Swimming down on one side of the pool, and back on the other. This is how you must swim if the lane has more than two people in it. It looks like this:
On the Bottom
Starting the set at the next :30 marker on the wall clock.
On the Top
Means starting the set at the :00 marker on the wall clock. It's often shouted with enthusiasm: "ON THE TOP!!!"
Split the Lane
This means each person gets half the lane: either the right or the left side. It's what is often used when the lane only has two people in it, so you don't have to worry about passing one another.