Sentences can be written in either the active or the passive voice.
In an active sentence the subject is carrying out the action.
The children climbed the tree.
In this sentence the children are doing the climbing, so it is an active sentence.
In a passive sentence the subject receives the action.
The tree was climbed by the children.
The subject (the tree) is not doing the climbing, but being climbed, so it is a passive sentence.
Sometimes the agent (the person or thing doing the action) is left out of a passive sentence.
Someone broke the window. (active)
The window was broken. (passive)
We could write 'The window was broken by someone' but it sounds clumsy and doesn't add anything to the sentence, so it is better to leave it out.