According to the Duffer brothers:
We don’t want to reveal much, other than to say it is an
interdimensional being that has more in common with the shark from
Jaws than Pennywise from It. When the monster enters our dimension,
it’s like a shark breaching the water. Very much like a shark, it
drags its prey back into its home, where it feeds. Each time it enters
our world, it leaves a small tear, or wound. That’s what Joyce found
in her wall in episode 4, and Nancy found in the tree in episode 5.
These tears are almost like portholes into the Upside Down. But they
don’t last very long. Like wounds, they eventually heal and seal up.
So every single portal but one is in fact made by the Demogorgon slipping between dimensions. That one, of course, is the first, permanent tear, which Eleven admits to making.
ELEVEN: The gate…I opened it. I’m the monster.
Presumably her involvement is why it is a proper gate, rather than a mere temporary tear.
Interestingly, the hole that Joyce found in her wall is described in a similar manner to the tears created by the Demogorgon: “That’s what Joyce found
in her wall in episode 4.” However, the Demogorgon quite clearly had not passed through that hole recently. Then again, it wasn’t quite a proper hole, but more of a barrier. Perhaps the Demogorgon was thinning the barrier between realities in preparation for crossing? Or perhaps Will was somehow…