As well as tenses and persons, Greek verbs have voices. They also have moods, which we shall come to later.
The two main voices are active and passive, so called because in the active the subject is represented as performing an action or being in a state whilst in the passive the subject is represented as acted on.
So the active λυω means I set free, whilst the passive λυομαι means I am set free.
Active – λυω τον ‛ιππον = I set free the horse
Passive – ‛ο ‛ιππος λυεται = The horse is set free