A seesaw (also known as a teeter-totter) is a long, narrow board which is fixed in the middle, a little way above the ground. When one end of the seesaw goes up the other goes down.
Children often like to play on seesaws. Two children sit on the seesaw, one at each end. The child who is down pushes the ground with his feet so that he goes up and his friend goes down. Then the friend pushes up so the first child goes down. They continue to go back and forth until they are ready to stop playing on the seesaw.
It works best with two children who are about the same size, or else the heavy person will make his end stay down. On some seesaws there is room for two or three children on each end. There is a handle for each child to hold on to so that they do not fall off.
In mechanics (a kind of physics) anything that is balanced in the middle and goes up and down like a seesaw might be described as something that "seesaws".