"Que" is used for quite a lot.

Basically, it acts very similarly to the English word "that", the relative pronoun. For example:

The book (that) I have is red.

El libro que tengo es rojo.

You'll notice that often times, in English, this word is omitted, but never in Spanish.

In addition to just "que", however, there are a few other relative pronouns in Spanish used for different situations. These include:

• El que/Los que
• La que/Las que
• El cual/Los cuales
• La cual/Las cuales
• Lo que/Lo cual

These are used in different situations, let's go over them:

El que, and it's plural/feminine forms are used when there is a bit of space between the noun being described and the pronoun. For example:

Encontré un libro, en el que hubo una llave.

I found a book, in which there was a key (or simply, "I found a book with a key in it")

The "el" will be changed if the noun is feminine/plural, like:

Allí está su mesa, en la que pone las flores.

There's her table, on which she puts the flowers (or simply, "There's the table she puts flowers on")

El cual, and it's plural/feminine forms are used when there is even more space between the noun and the pronoun. For example:

Acabo de comprar un nuevo apartamento, cerca del cual hay un restaurante fantástico!

I just bought a new apartment, near which there's a fantastic restaurant!

Similarly, "el" changes to its feminine/plural form based on the noun being described. In addition, when it changes to plural, "cual" becomes "cuales".

Lo que and lo cual are slightly more complicated. These refer to an idea or situation, not a physical object. Lo que is similar to the English word "what" in certain situations. For example:

Lo que hiciste ayer fue horrible.

What you did yesterday was horrible.

The "que" changes to "cual" when the idea or situation has already been mentioned. For example:

Cuando era niño, siempre cocinaba la cena, lo cual a mi mamá le encantaba.

When I was a boy, I always cooked dinner, which my mom loved.

As you mentioned in another comment, you already know how to compare, in situations like:

Él tiene más dinero que José.

There are a few situations, as well, where que is used after a verb and before an infinitive. For example, "tener que" and "hay que". These express varying levels of an obligation, like in the following sentences:

Yo tengo que hacer la tarea.

I have to do homework.

Or

Hay que beber mucha agua.

One must drink a lot of water. (This is impersonal, hence the usage of "one".)

Anyway, I hope this helped you out.