Here's what the letter contains:

Dear Messrs Chairmen:

In previous congressional testimony, I referred to the fact that the Federal Bureau of Investigation (FBI) had completed its investigation of former Secretary Clinton's personal email server. Due to recent developments, I am writing to supplement my previous testimony.

In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation. I am writing to inform you that the investigative team briefed me on this yesterday, and I agreed that the FBI should take appropriate investigative steps designed to allow investigators to review these emails to determine whether they contain classified information, as well as to assess their importance to our investigation.

Although the FBI cannot yet assess whether or not this material may be significant, and I cannot predict how long it will take us to complete this additional work, I believe it is important to update your Committees about our efforts in light of my previous testimony.

Sincerely yours,

James B. Comey

Director

The important bit is here:

In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation.

In other words, investigators realized that some of the emails they reviewed for Clinton's case may be pertinent in an unrelated case. The rest of the email goes on to explain that the FBI wishes to review these emails in question to determine whether or not that contain classified information and if they are indeed useful to this unrelated case.

There's nothing here that suggest that Clinton's case should be or is re-opened, he even repeats that the FBI has completed their investigation in regards to RHC's personal email server.

In other words he saying 'Hey so we got this other case over here... and some of the stuff we looked at in HRC's case may be useful... so we're taking a look at it again, just letting you know.'

He doesn't mention what this other case is, only that its unrelated... So until we get more information, i'd say its not worth worrying about, let the trumpets have their fun.