MixinBootstrap is an unofficial way to boot the Mixin library in a production Minecraft Forge environment. You need it when another mod depends on Mixin and your Forge build does not load it on its own.
It runs on Forge for Minecraft 1.12.2 - 1.17.1. The author notes that Mixin ships with Forge 1.15.2-31.2.44+ and 1.16.1-32.0.72+, and that no support is offered.
When a mod on your Minecraft server asks for Mixin, this dependency goes in with a single click from the panel.