[Felipe]

You use it when the direct object of the sentence is a person.

For example, I found him in a store. Encontré a él en una tienda.
I looked for you. Busqué a ti.

There's no english equivalent, so sometimes it's difficult to understand. The a comes in front of the direct object, as you can see. But it's only used for a person. If I found a garbage can in the store, I wouldn't use personal a.

Encontré un basurero en la tienda.

But a is also a preposition, different from the personal variety. It can translate as "to" in english. Fui a canadá. I went to Canada. Personal a doesn´t translate like this.

You can also use it with things towards which the owner has personal feelings. I might use it for my dog or cat, for example.