Quick explanation:
The Spanish verb hacer means "to make" or "to do". This verb is commonly used in Spanish when talking about the passage of time.

Fue en diciembre. Hace dos meses.
It was in December. It makes two months.
or
It was In December. Two months ago.

Están divorciados desde hace un año.
They are divorced since it makes one year.
or
They have been divorced for a year.

¿Cuánto tiempo hace que él está leyendo?
How much time does it make that he is reading?
or
For how long has he been reading?

Long explanation:
Let's use the following formula to talk about time in the present:

action in the past

hace + time + que + preterite tense

Hace tres años que Mario al concierto de Ricky Martin.
It's been three years since Mario went to Ricky Martin's concert.
or
Mario went to Ricky Martin's concert three years ago.

Hace dos semanas que me operaron.
It's been two weeks since I had a surgery.
or
I had surgery two weeks ago.

action started in the past continuing in the present

hace + time + que + present

Hace cuatro días que no te lavas el pelo.
It's been four days since you don't wash your hair.

Hace 8 años que no vemos a mamá.
It's been eight years since we don't see mom.

Note: The order of the use of "hace" can be at the beginning or end of the sentence. Look at the examples:

Hace cinco horas dejamos a Melissa donde Gaby.
We left Melissa at Gaby's five hours ago.

or

Dejamos a Melissa donde Gaby hace cinco horas.
We left Melissa at Gaby's five hours ago.