You can read the release notes here.

From the support page:

Impact: Visiting a maliciously crafted website may lead to an unexpected application termination or arbitrary code execution

Description: Multiple memory corruption issues existed in WebKit. These issues were addressed through improved memory handling.

And:

Impact: Inconsistent user interface may prevent users from discerning a phishing attack

Description: A user interface inconsistency existed in Safari that allowed an attacker to misrepresent the URL. This issue was addressed through improved user interface consistency checks.