Your sentence is the passive version of this sentence :
'Heavy traffic causes a lot of pollution.'
In the active sentence above, 'Heavy traffic' is the subject and 'a lot of pollution ' is the object.
In your passive version of the sentence, 'A lot of pollution' is the subject and 'heavy traffic' is the agent. The agent is the thing that CAUSES the action to happen in a passive sentence. The agent is always connected to the sentence with the preposition 'by'. Here are some more examples:
Shakespeare wrote 'Hamlet' : active
'Hamlet' was written by Shakespeare : passive
The Egyptians built the pyramids : active
The pyramids were built by the Egyptians: passive