Earlier this week, the ouroboros crate, a dependency of leptos_reactive, identified a fundamental soundness issue. This was fixed immediately in Leptos on the main branch by switching to self_cell instead. However, there have been a number of breaking changes on main since 0.3.0 in preparation for 0.4.0, so main could not be released as 0.3.1. There are a number of other changes that need to be made before 0.4.0, so it did not make sense to immediately release a new major version.
This patch release just rewinds to 0.3.0 and replaces ouroboros, without including any additional features or changes since then. If you have already been using main, you should continue using main. If you have been using 0.3.0, you should update to 0.3.1.
See additional discussion here.