From what I've been able to piece together it sounds like it's some emails that may have been from Hillary or she was just included. At the time that Comey sent the message he didn't actually know what was in the emails because the FBI hadn't obtained a warrant to look at the contents of the emails. So, they could be new emails or they could be duplicates of what has already been investigated. If the emails are new then they have to determine if there was classified info.
Right now, this isn't the smoking gun. It might be but we probably won't know for a couple months.
Comey couldn't just sit on this information and not share. It's an impossible position to be in with the election less than two weeks away.