tread the boards

From the fact that theatre stages are often made of wooden boards which are trodden by actors. Compare French monter sur les planches (literally “to get up onto the boards”).

The term boards (“a theatrical stage”) was first attested in the mid-1700s, this idiom itself was first attested in the mid-1800s, and was preceded by the idiom tread the stage, first attested in 1691.