For people who know rust, here is a diff for 0.17.3 (what the footer says) and 0.17.4: https://github.com/LemmyNet/lemmy/compare/0.17.3...0.17.4
It has some database optimizations and changes to the "hot" ranking algorithm.
The release notes (https://github.com/LemmyNet/lemmy/releases/tag/0.17.4) say that the database migration can take a while, so there might be downtime involved in the deploy. But it also says it fixes security issues, so that should probably be prioritized.