What are SSL heartbeats?

Heartbeat is an echo functionality where either side (client or server) requests that a number of bytes of data that it sends to the other side be echoed back. The idea appears to be that this can be used as a keep-alive feature, with the echo functionality presumably meant to allow verifying that both ends continue to correctly handle encryption and decryption. The problem, of course, is that until the recent patch, OpenSSL did not guard against sending back more data than was provided in the first place. I'm not really aware of where the heartbeat extension is actually used in an application since most communication that requires it (e.g. websockets) rely on their own keep-alive features implemented on a higher level.

I can't answer your second question---but it would surprise me if the answer was yes.

Try this for info on heart beats:http://www.troyhunt.com/2014/04/everything-you-need-to-know-about.html

I'm not really an apache guy, I gather that the flag works but that there may be a performance hit. The advice is to recompile. Also talk to your devs about sending emails, you may want to consider asking your users to change their passwords - just to be on the safe side. I've had a couple of services email like that already

You can find all the details on the TLS and the DTLS Heartbeat extension by reading RFC6520: https://tools.ietf.org/html/rfc6520.