Prepositions are used to link words. They don't have a meaning on their own. The preposition a is used in many different ways. Here are a few of many examples:

going to + infinitive
Voy a lavarme las manos.
I am going to wash my hands.
María les va a dar el dinero.
María is going to give them the money.

to introduce people
Te quiero presentar a una amiga.
I want to introduce a friend to you.
Juan, te presento a María.
Juan, let me introduce you to María.

to express where you are going or to whom something is aimed at
Fuimos a Barcelona en avión.
We went to Barcelona by airplane.
Él va a su casa.
He goes to his house.
Nosotros visitamos a nuestra abuela.
We visit our grandma.

to accompany certain verbs
No me acostumbro a la vida universitaria.
I don't get used to college life.
No nos atrevimos a decir nada.
We did not dare to say anything.

to express what time an event ocurred
No pude acostarme a las 9 porque tenía muchas tareas que hacer.
I could not go to bed at 9 because I had too much homework.