Ir, er, ar. What are they?
In Spanish there are 3 verb types: -ar, -er, and -ir. For example, "hablar", to speak, "comer", to eat, and "vivir", to live.
Like m.tastic said, they are the verb endings for Spanish verbs.
The verb ending can tell you how to conjugate the verb. There are a few exceptions (irregular verbs) but most (regular) verbs follow patterns.
For example, to conjugate an -ar verb like hablar, the verbs are
Yo hablo (o)
Tú hablas (as)
Ud/él/ella habla (a)
Nosotros hablamos (amos)
Ustedes/ellos/ellas hablan (an)
All regular -ar verbs will end just like hablar. For example, nadar (to swim), caminar (to walk), and cantar (to sing):
Yo nado, camino, canto
Tú nadas, caminas, cantas
Ud/él/ella nada, camina, canta
Nosotros nadamos, caminamos, cantamos
Uds/ellos/ellas nadan, caminan, cantan
Once you know how to conjugate one -ar verb, you pretty much know how to conjugate them all.
Likewise, once you know how to conjugate one -er and one -ir verb, you know how to conjugate most of the others.
Like I said, there are a few exceptions, but even the exceptions fall into similar groups. It's confusing at first but eventually it will all start to be second nature and the verbs will just sound right when they follow the pattern. You won't really even think about it.