Most astrologist today work upon a set theory that believe the location of the planet today in relationship to the location of the planet on the day you are born would have an influence on you and the same apply to everyone in the world and they had a set of theory of how each planet would effect you under each circumstances. And it is base on that set of theory that Astrologist create theory in regards to your future.
Sometime, Horoscope do tend to have a correct prediction towards the future, sometime more accurate, sometime less accurate, sometime 110% wrong.
So, I won't trust it completely, but I won't ignore it completely either. I tend to look at the negative things they say and keep an eye out for it as for the rest I don't look at it.