Fro is a short, old-fashioned word meaning “away.” You almost never see it by itself anymore, but it survives in the phrase to and fro, which means back and forth.

When something moves to and fro, it goes one direction and then returns, over and over. A pendulum swings to and fro. Ocean waves roll to and fro on the beach. A father might pace to and fro in the hospital waiting room, walking back and forth nervously.

The phrase captures that restless, repetitive movement: not going anywhere in particular, just moving back and forth along the same path. If you're rocking in a rocking chair, you're moving to and fro. If you're watching a tennis match, your head turns to and fro as you follow the ball from one side of the court to the other.