An irregular verb is one which does not follow standard rules of conjugation. There are three types of irregular verbs in the future indicative tense: (er) verbs whose infinitives drop the "e", (er) verbs whose stems end in l or n change the "e" of the ending to "d", and those verbs with completely irregular stems. Let's look at some example sentences:
No podré ir a la barbería.
I will not be able to go to the barbershop.
Te dirá que te quedes.
He will tell you to stay.
¿Para cuándo la querrás?
For (by) when will you want it?
Cuanto más estudie, más sabrá.
The more you study, the more you will know.
Habrá un contrabandista cruzando la frontera a la medianoche.
There will be a smuggler crossing the border at midnight.
Con levantarnos a las diez, habremos descansado bastante.
Getting up at ten, we will have rested plenty.