In short, Heartbeat allows one endpoint to go "I'm sending you some data, echo it back to me". You send both a length figure and the data itself. The length figure can be up to 64 KiB. Unfortunately, if you use the length figure to claim "I'm sending 64 KiB of data" (for example) and then only really send, say, one byte, OpenSSL would send you back your one byte -- and 64 KiB (minus one) of other data from RAM.
!Whoops
This allows the other endpoint to get random portions of memory from the process using OpenSSL. An attacker cannot choose which memory, but if they try enough times, their request's data structure is likely to wind up next to something interesting, such as your private keys, or users' cookies or passwords.
.None of this activity will be logged anywhere, unless you record, like, all your raw TLS connection data
.Not good
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