There are two voices, active and passive. In active voice, the subject does the main verb's action, and in passive voice, the subject is the recipient of the verb's action.
Compare: Active: The lion eats the deer. - The subject (the lion) does the action (eats) Passive: The deer was eaten by the lion - The subject (the deer) is the recipient (was eaten)
It's usage is often to "hide" the subject of the sentence if it were in active voice. In active "I made a mistake", "I murdered somebody" to passive "A mistake was made [by me]" "Somebody was murdered [by me]" with [by me] not being needed.
In Swedish, passive is more common and its constructions can be read about here: https://en.wikipedia.org/wiki/Swedish_grammar#Passive_voice.
There can be confusion between passive and deponent verbs, as Arnauti mentioned, because one way to create the passive is to add -s to the infinitive:
Active: Tjuven dödar mannan. = The thief kills the man. Passive: Mannen dödas av tjuven. = The man is killed by the thief.
versus a deponent verb like "minns" Active: Jag minns ordet = I remember the word.
Note I'm a native English speaker, not Swedish, so I hope I got that mostly correct.