To and fro means moving one way and then back again, over and over: a swing, a boat on waves, a person walking up and down a room while worrying. 'Fro' is an old word for 'back' that survives nowhere else in modern English — it lives only inside this phrase, which is why it looks so strange on its own. Most often it comes after a verb of movement: the branches swayed to and fro, she rocked the baby to and fro. It feels a little old-fashioned and a little literary; in ordinary conversation people are more likely to say 'back and forth', which means exactly the same thing. With hyphens, to-and-fro can work as a describing word before a noun (a to-and-fro motion) or as a noun for the whole business of going back and forth — and that noun often means an exchange of words rather than real movement: after a lot of to-and-fro between the two lawyers, they agreed. That last use is common in news and business writing, where the to-and-fro is the arguing, the emails, the offers going one way and answers coming back.