Lock (water transport)
The lock is a place where boats that travel up or down a river or canal can be moved to the next higher or lower level.  Locks are built in places where the level of the water in the river or canal suddenly changes.  This may be because of a waterfall there, or because a dam or a weir has been built, or because some other thing is in the way.  The lock is like a big chamber with gates at each end.  They have lock gears which empty or fill the chamber with water.  Locks help a river to be more easily navigable (easier for boats to travel up and down), or for canals to be built across country that is not level.