Experimental branches are Git branches (marked as draft) which are not immediately destined to be released in the main code base and thus, are a good way to make tests, show a proof-of-concept, which when ready, can be merged into trunk, for release in the next version of Tiki.
This is a similar idea, but different than features which are part of the main Tiki code base but tagged as Experimental.
Experimental branches are great in the early stage of development because you can change directions without consequence. Once you merge into trunk (since some people are running sites on trunk), you need to handle the Database Schema Upgrade. On the other hand, things don't get tested as much in experimental branches. So eventually, you've got to merge to trunk/master 😊