This is usually used to mean “outer space” in general, but it technically means “between stars” or “between solar systems.”
Most things whizzing through space are in an orbit of some kind. They’re either orbiting a planet (like a moon) or a star (like the planets around our sun). Something that is “intersellar” is not orbiting anything. Rather, it’s traveling across star/solar systems – flying through them and coming out the other side.
This is actually pretty rare (see below). I assume anything truly interstellar would need massive velocity and energy to avoid getting sucked into the orbit of whatever it flew past (and some interstellar objects probably cease being interstellar when they fly past something big enough to suck them in).
Why I Looked It Up
I found this article: US military confirms an interstellar meteor collided with Earth
An interstellar meteor is a space rock that originates from outside our solar system – a rare occurrence.
Oumuamua is a known interstellar object detected passing through the Solar System. It is possibly the second interstellar object known; the first being a purported interstellar meteor that impacted Earth in 2014.