Every body is losing their shit but they are all completely confused and wrong about what is going on, so I'll give you the correct answer that everybody is unwilling to say because it doesn't support the "Hilary is a stupid criminal bitch" narrative they want to push.

The staffer we are talking about, likely down to legal advice, decided to plead the fifth in front of congress. People are assuming this means he is hiding something but that's just plain wrong. This is pretty standard legal advice even when you are completely innocent. You don't want to do/say anything incriminating.

Having pleaded the fifth, the staffer can be ordered to testify if a judge determines that his testimony is necessary to the public interest. This is a very low bar requirement. Basically if the judge thinks you're guilty of having done something or thinks you have information relevant to somebody having done something, they can force you to testify. This wasn't done, which suggests that they don't really think he has relevant information.

Interestingly, and it seems to be consistently overlooked, Clinton herself told the staffer to testify. She wanted him to give information over. It's very likely the only reason he didn't (yet) is because he was told by legal council to get immunity before divulging any information since, again, it's always advisable to not incriminate yourself even when you're sure you haven't done anything wrong.

So long story short, none of this new information is likely to be indicative of any big thing. To the uninformed it sounds significant when you hear that somebody was "granted immunity" or that somebody felt compelled to "please the fifth" but it's basically just standard operating procedure for these kind of investigations. We will have to wait and see if any new information comes out but this specific inside is genuinely pretty uninteresting or inconsequential.