What does listo mean in English?

"Listo" can mean "ready."

He is ready. = Él está listo.

She is ready. = Ella está lista.

"Listo" can also mean "clever."

He is a very clever boy. = Él es un chico muy listo.

She is a very clever girl. = Ella es una chica muy lista.

Note: the definition is dependent on which verb is used. To express that someone is ready, use the verb "estar." To express that someone is clever, use the verb "ser."