What is a past participle?
A past participle indicates past or completed action or time. It is often called the 'ed' form as it is formed by adding d or ed, to the base form of regular verbs, however it is also formed in various other ways for irregular verbs.
It can be used to form a verb phrase as part of the present perfect tense.
I have learnt English. (Learnt is part of the verb phrase 'have learnt')
It can be used to form the passive voice.
Her hair was well brushed.
It can also be used as an adjective.
As an adjective: He had a broken arm. (Broken is used here as an adjective.)
Here is a comprehensive list of irregular verbs.