
Heartbleed is a software bug in the open-source cryptography library OpenSSL, which allows an attacker to read the memory of a server or a client, allowing (with special forget packets) reading (small) portition of the victim client. This could expose to lost of some data and potentially also confidendial data.
Heartbleed.com has a detailed explanation of the issue, which is related to the “heartbeat” section of OpenSSL’s transport layer security (TSL) protocols and has been in the wild since March 2012 and affect all version from OpenSSL 1.0.1 through 1.0.1f.
You need to upgrade the OpenSSL binaries if you are implementing services with SSL but also you have to change your password (or you certificates) if you are using (of have used) affected services (see also The Heartbleed Hit List: The Passwords You Need to Change Right Now).