It's really self explanatory.
Doesn't allow players to build below a certain y value, defined in the config.
If they try, they will be sent the message that is also defined in the config and the block will not be placed.
Made for this request:
https://www.spigotmc.org/threads/min-build-height.93381/page-2

MinimumBuildHeight 1.0
Set the minimum build height of your world