The past participle is part of a verb form used to create compound tenses. You're already familiar with how the past participle of regular verbs is formed.
For irregular verbs, the past participles also usually begin with a prefix, ge- but they don't have a -t at the end. Instead they end with -en.
 

Frequently there is also a vowel change in the stem. It's easiest if you learn the irregular verbs along with their past participles.

Examples:
trinken – ich habe getrunken
liegen – ich habe gelegen
saufen – ich habe gesoffen
gehen – ich bin gegangen
 

Past participles of inseparable verbs
Verbs that have no emphasis on the prefix - in other words that cannot be separated - are called inseparable verbs. Their past participles are formed without the prefix ge-. Regular verbs end with
-t and irregular verbs end with -en.

There can also be a stem change in inseparable verbs if they are irregular:

verstehen – Ich habe dich verstanden.
 

"sein" or "haben"?
The perfect tense of most verbs is formed with the helping verb "haben". For the most part, they are verbs that can take an accusative object.

Example:

Some verbs require the helping verb "sein" to form the perfect. These are mainly verbs that involve a change of location or condition. The helping verb is conjugated in the present tense and the past participle remains unchanged.

Examples:

More:

The past participle of separable verbs
Verbs that have an emphasis on the prefix are called separable verbs because the conjugated form can be separated from their prefix. You're already familiar with such verbs. Their past participles are formed differently than normal verbs. With separable verbs, the ge- comes after the separable prefix.

The participle ending is -t for regular verbs and -en for irregular verbs.

There can also be a stem change in separable verbs if they are irregular:

aufstehen – ich bin aufgestanden
losgehen – ich bin losgegangen