Apple continues to upgrade and improve on its new browser, Safari.
You can get this update via Software Update or download it from the link above.
From the release notes...
The Safari Update 2-12-03 improves the compatibility with popular web sites based on Safari user feedback, further improves the performance of loading web pages and Flash content, adds support for XML, increases standards conformance and delivers improved application stability. The update also enables access to web sites that offer self-signed security certificates.