SSL (Secure Sockets Layer) and TLS (Transport Layer Security) are both designed to provide security to web connections. TLS is the successor protocol to SSL, though both are often referred to as ‘SSL’. The last version of SSL was SSL version 3 (SSLv3), which is no longer deemed to be secure due to a vulnerability dubbed POODLE. Similarly, TLS version 1.0 is also no longer considered to be secure, as some implementations are vulnerable to POODLE, and cryptographic vulnerabilities have been found in the underlying RC4 cipher. All versions of SSL, and TLS 1.0 have been superseded by TLS 1.1 and 1.2, the latter of which is the current recommendation.
When visiting a secure web page, the browser and web server negotiate to use the most secure version of SSL/TLS supported by both parties. In practice, this means that SSLv3 is rarely used to provide security. However, certain browser behaviour allows a man-in-the-middle to downgrade the negotiated protocol to SSLv3 or TLS 1.0, after which they can perform an attack. The Extension indicates whether the web server supports SSLv3, which could mean that a downgrade attack is possible.