What is the past participle?

The past participle is an important part of the verb and is used in some of the perfect tenses.
Take a look at the following regular verb:
walk (simple form)
walked (simple past)
walked (past participle)

Take a look at the following irregular verb:
 See (simple form)
Saw (simple past)
Seen (past participle)

Section D: The Perfect Tenses

Present Perfect She has seen him once already. Used for activities that happened at an unspecified time in the past.
  I have looked over the report five or six times already. Used for repeated activities in the past.
  I have been here since six this morning.

I have been here for six hours.		 Used for situations that started in the past but continue now (with for and since)
  has / have + past participle  
Present Perfect Progressive He has been sleeping for 11 hours. Used for expressing the length of time an activity has been taking place.
  has / have + been + verb + ing  
Past Perfect She had read the book before the professor assigned it. Used for an activity that occurred before another activity in the past.
  had + past participle  

This may seem a bit overwhelming but remember this is to be used as a reference guide. If you need activities to help your learner with some of these verb tenses, check out the grammar textbooks by Azar and Murphy.