A release build of OpenWrt is a stable version of the firmware. It is expected to be supported for the long term. It is safe to use in production or with your family.
A snapshot (or trunk) build is a version of OpenWrt that is “in development”. These are rebuilt frequently, often multiple times a day. Although they are the latest version, there is no guarantee that any particular snapshot build will be bug-free, or even work at all.
Find more about this is on page about development builds / snapshots.
The OpenWrt version history page mentions releases, snapshot and branch logic.