Verbs usually have an active subject. For example, in the sentence ‘The dog bit the man’, the dog is the subject of the verb; the dog is the one that is actively doing something (the biting). The sentence can be rearranged as ‘The man was bitten (by the dog)’ – it is now a passive sentence; the subject is now the man but the man is not doing anything active.
Here is another example:
I wrote the poem. (active) The poem was written (by me). (passive).
As in English, you form the passive in French by using the verb être (to be) in the correct tense, followed by the past participle.
La circulation a été réduite. - Traffic has been reduced.
Les sacs seront réutilisés. - The bags will be re-used.
La pollution est causée par les gaz d’échappement. - Pollution is caused by exhaust fumes.
Note that the past participle is treated as an adjective and has to agree with the first noun.
For example, la pollution is feminine so you need –e on the past participle causée. Les sacs
are masculine plural and so you add –s to the past participle réutilisés.