It's not a simple question to answer, you may read what little information has been gathered here & here, it is only recently been "discovered" by doctors & a treatment is not yet clear. But I can tell you better than any professional can because I have lived it for eight years; half of my life. Although personally I do not care much for the term "daydreaming" to describe this as I feel it cheapens its severity.

In my own words what I have done (and do) is I pretend that I am living another life, not as myself, but as someone I feel comfortable being. I have imaginary friends, an imaginary life, complex & elaborate beyond your wildest dreams.
Most of us with this disorder have repetitive motion involved with this as we fantasize, for me I act it out physically & often talk aloud (though quietly enough so that no one will hear).
If I am not alone I will confine it to my head, imagining that I am talking to one of my make-believe characters instead of a real person.

I have done this ever since I was at least eight & possibly before that.
It exploded when I was nine & ten, when most of my entire world became a fantasy, most of my time I spent pretending.
I typically fantasize in real time, as I go about my day I will pretend that I am my other self going about my day. But as you can tell, I am not fantasing now, I am writing as myself.
I will always write as myself here.

Now, I have to make clear that maladaptive daydreamers do not lose touch with reality, it is not psychosis, it is not schizophrenia, we do not literally hear & see things.
We are fully in contact with reality, but we use our fantasies as a way to escape whatever unpleasentless may be our reality. We know it is not real, we often deny that it is a real problem & think it is phase that we'll grow out of. But when or if we ever decide to try to live as ourselves & have found that living as us in the moment that is overwhelming & uncomfortable for us as we use fantasting as a coping method. It is a humiliating condition for us & I was mortified when I first admitted it to myself.