Believe it or not, it’s the same as the past participle in English:
Hemos hablado.
We have spoken.
When used in this way (to form the perfect), it does not agree in number or gender.
In the following uses, it does agree in gender and number:
As in English, it can also be used as a *passive* participle, to form the passive voice.
El auto fue robado.
(Plural: Los autos fueron robados)
The car was stolen.
In reality, however, Spanish tends to use other constructions instead of the passive voice:
Robaron el auto.
Alguien robó el auto.
Se robó el auto.
As in English, it can be an adjective or predicate adjective:
Se encontró el auto robado.
The stolen car was found.
Pedro está aburrido.
(María está aburrida.)
Peter (María) is bored.
Pedro es aburrido.
(María es aburrida.)
Peter (María) is boring. (Note that this is a bit idiomatic.)