Here's how it breaks down:
* montar: To ride, mount, or assemble.
* tu: You (informal, singular)
* montas: You (informal, singular) ride, mount, or assemble.
Examples:
* Tú montas en bicicleta. (You ride a bike.)
* Tú montas un rompecabezas. (You assemble a puzzle.)
* Tú montas a caballo. (You ride a horse.)