The SSL checker mini-bot gives you information about validity, expiration date and potential problems with the SSL certificate of the domain you provide.
SSL stands for Secure Sockets Layer, which is a cryptographic protocol designed to provide secure communication across the Internet. SSL uses X.509 certificates (asymmetric cryptography) to assure the counterparty whom they are talking with, and to exchange a symmetric key. This session key is then used to encrypt data flowing between the parties. This allows for data/message confidentiality, and message authentication codes for message integrity and as a by-product message authentication.
Several versions of the protocols are in widespread use in applications such as web browsing, electronic mail, Internet faxing, instant messaging and voice-over-IP (VoIP).
An important property in this context is perfect forward secrecy, so the short term session key cannot be derived from the long term asymmetric secret key.
More info about SSL on Wiki.