El's mother was a subject of MK ultra, a government experimentation where they tried brain control (this was a real thing) as a result she had a baby with powers, El's powers are psychokinesis and Astral projection, her power of astral projection allows her to see people in different places, including the upside down, and her power of psychokinesis allowed her to open, and close, the door to the upside down, the only other person who was a child of MK Ultra, in Stranger things, is 8, she only has the power to project hallucinations as far as we know, so it seems like El is the only one who has these powers, at least the only one we know of,
it seems that they stumbled upon the upside down when El was astral projecting once when they had her spying on someone (presumably a Russian agent). Since then they tried to get El to explore the upside down to see if they could somehow use it as a weapon. El accidentally opened the gate when she got scared while projecting.
the upside down is a shadow universe, one which is an alternate universe of our own, this means that our universe and the upside down share a lot of similarities but have differences, including possible differences in properties, if you ever played D&D this is like the Veil of Shadows, and is even compared to it in the show
In the Multiverse theory of multiple universes there can be two, or more, universes occupying the same space, this seems to me to be the type of multiple universe they at trying to show with the upside down, our universe and the upside down seem to occupy the same space but most of the time things do not bleed over from one to the other, it took El opening the "gate" to allow this to happen, I hope that helps