Redjake

I've heard the term used a lot and I don't know what it is. I've tried doing research on it online but I can't get a clear cut answer. What does "Post-Crisis" mean? I hear people always say things like "yeah, Batman, post-crisis, can blah blah blah" or "Superman can't do blah blah blah now, post-crisis" and "are we talking post-crisis Superman?"

What crisis are they talking about?

Comic book nerds help me out here!

Thanks in advance.

__________________
Off the record, on the q.t., and very hush-hush.