From the Mac App Store update page:

The Safari 7.0.3 update is recommended for all OS X Mavericks users and contains improvements to compatibility, stability, and security. This update:

• Fixes an issue that could cause the search and address field to load a webpage or send a search term before the return key is pressed
• Improves credit card autofill compatibility with websites
• Fixes an issue that could block receipt of push notifications from websites
• Adds a preference to turn off push notification prompts from websites
• Adds support for web pages with generic top-level domains
• Strengthens Safari sandboxing
• Fixes security issues, including several identified in recent security competitions

Here’s a link to the details of the security contents of the upgrade.