Each Bitcoin you send has a "script" which says how it can be spent: usually just a signature for the owner's key. There's lots of other stuff yo can do though.

Back in 2010, Satoshi ripped out a pile of Bitcoin script because it had no limits and you could make coin spends which would blow up nodes.

Putting reasonable limits in means we can put all those back, so you can make your Bitcoin do fancy tricks, if you want (need?).