Apple’s patch notes:
The macOS High Sierra 10.13.3 Update improves the stability and security of your Mac and is recommended for all users.
- Addresses an issue that could cause Messages conversations to temporarily be listed out of order
- Resolves an issue that could cause your Mac to stop responding when connected to an SMB server
The security patch notes for macOS High Sierra 10.13.3 specify 17 different security holes fixed in the update. They include a variety of kernel, low-level areas, audio, and WebKit issues, many of which can lead to a compromised Mac.
You can download the update through the Mac App Store.