Unfortunately it can mean a few things:
The question is being asked during the event, and the information is incomplete. While the event evolves, the question or answers may change as new information becomes known, or existing information becomes more clear. The OP and others are actively participating in keeping the question as up to date as possible. In this case the notice should be remove once the event is "complete"
The question needs an immediate answer, late answers are not useful (this one is new to me - I'd rate this as very high on the "too localized" scale)
There may be more ways to view it, but I suspect it largely depends on the site it's on, and in some cases both may apply - although there are few cases where post analysis is truly unwelcome, so if anything the second definition (in addition to being too localized) is much weaker and less useful.
For instance, a question posed on skeptics concerning the news/propaganda of a given nation during a coup would be noted. People who read the question, or attempt to answer it or edit any of the posts should be aware that things are in a state of flux, and if they do post they should keep an eye on the situation and their post so as to avoid being wrong when it turns out the situation changed underneath them.
We can probably come up with contrived examples of questions that might require this notice for each site, "Why is this implementation of -somehash- weak?" would be noted as a current event on Stack Overflow (or the cryptographic site) if someone announced that it was broken, but their analysis has not yet been fully released (say, just days before defcon).
A post about derailleur failure modes on bicycles might be a current event if that was the most likely cause of a crash during the tour du france.
It's essentially, "This post is related to an evolving event. Participants should follow up frequently during the event and update their posts as needed."