The federal government has given former State Department employee Bryan Pagliano an immunity agreement to get him to cooperate in the investigation of Clinton's use of her private email server. Here's some background on how that works and what it means.
Criminal investigations such as this one can be sensitive, delicate affairs. In more "traditional" cases such as assaults and bank robberies, the people with information--for example, victims and bystander eyewitnesses -- have no fear of legal exposure themselves and are typically willing to help investigators however they can. By contrast, in the white-collar and governmental conduct contexts, many of the people the investigators will want to talk to will be close enough to the situation to have their own concerns about getting caught up in the investigation (regardless of whether they actually believe they've done anything improper).
A lawyer representing such a person has to be extremely cautious. There's no general obligation to "cooperate" with government agents requesting interviews at the investigatory stage. And especially in this type of case, where guilt or innocence can often hinge not on what physical conduct took place but on various individuals' motives and who knew what when, a "cooperating" person's statements can end up constituting extremely damaging evidence against that person. (There's a reason why the Miranda warnings don't say "anything you say can and will be used in your favor.")
Many years ago, Supreme Court Justice Robert Jackson said that "any lawyer worth his salt will tell [a] suspect in no uncertain terms to make no statement to police under any circumstances." And that logic isn't limited to formal "suspects." Especially in the early stages of an investigation, no one knows who the "suspects" may end up being, so the most prudent course is often remaining silent and declining any interview requests.
Even a subpoena -- which, unlike a simple interview request, is generally treated as an enforceable demand for a witness to give testimony -- can be overridden by a witness's self-incrimination concerns. If there's a reasonable chance that a witness's testimony could constitute a "link in the chain" of evidence needed to convict him of some criminal offense (even if the witness hasn't been charged or even identified as a suspect), the witness can invoke his Fifth Amendment privilege and refuse to testify. Doing so doesn't require the witness to acknowledge guilt; it's entirely legal for someone who denies guilt to nonetheless refuse to testify out of a fear that the testimony could be used against him.
So when the government wants information or testimony from a witness who has legitimate concerns about giving it, a delicate dance begins. The only way the government can force such a witness to testify is through a formal grant of immunity promising that nothing the witness says can be used against him. This immunity has to extend beyond direct use (for example, introducing the witness's statement against him at trial) to "derivative use." So, for example, if the immunized witness tells the government about a damaging document or about another witness who could incriminate him, the government can't use that evidence unless it can demonstrate that it would have found that evidence on its own even if it had never talked to the immunized witness.
The government generally doesn't like giving out immunity agreements. Doing so tends to largely insulate the immunized person from prosecution -- although the government can still prosecute based on evidence it developed independently, proving such independent development can be challenging, so there's a real chance that immunizing a witness renders him effectively unprosecutable. Additionally, immunizing the witness makes him a less effective prosecution witness if the case ever goes to trial against the target, whose defense lawyer can cross-examine the witness on issues such as why he thought he needed to be immunized.
For those reasons, the government often tries to get a witness's testimony through something short of formal immunity. One option is a "proffer agreement," through which a witness is interviewed with an understanding that the statements made during the "proffer session" won't be used against him. This is significantly less protective than immunity. Not only do proffer agreements typically state that the government can make "derivative use" of the statements, but modern federal proffer agreements are riddled with exceptions. They typically state, for example, that the government can use the proffer statements against the proffering witness if he testifies in his own defense at his trial, or even if at trial he takes any position "inconsistent" with his proffer statements (which can arguably be as little as pleading "not guilty"). Defendants and their lawyers may accept proffer agreements if that's the best they can do and they're willing to accept the associated risks, but they're definitely not preferred.
Here, the government has given Pagliano actual immunity. It's generally known within the criminal defense community that immunity agreements are rarely offered, and that when they are it's because the government is highly interested in what the immunized witness has to say. Pagliano's lawyer did well to get him this agreement -- we'll have to wait and see what Pagliano ends up saying.