How can a federal judge put a halt on an executive order the president created?

It doesn’t make sense to me. I thought the president had the final word on everything. I’ve been reading about this and no one understands how this is even possible.

For those who don’t know, a Seattle judge put a temporary lift on the ban preventing people from the middle east from entering the U.S.

I would like to add this: I do NOT want political discussions on what you think of the ban in the first place. I just want to know how it’s possible for a judge to do something like that. Please don’t argue, or add personal opinions regarding Trump, etc. Thank you.