In bossfights logic, it goes like this

Universe is the smallest, and easiest to travel.

Multiverse is a bit more difficult, but still a common ability

Then there's white space, which is not very common. Even the collector had some difficulty trying to access it

After that there's dimensions and timelines, both accessible with a dimensional diamond obtained from white space. Collector can access other timelines, but doesn't because it would put this one in danger. Similarly with dimensions.

Somewhere in between all of that are realms only specific characters can access, such as the realm of creation, which belongs to the Collector

Then there's things only the Traveler can access, those being, realities

that doesnt explain what they are though...

A universe is basically what we're in right now

Multiverse is what universes are held in, and are quite difficult to travel between

Whitespace is an endless void that exists... Somewhere. I'm not completely sure about it.

Timelines are our way of keeping AU's separate, but dimensions are a thing eggy brought in so ask him.

And then of course there's realms only specific beings can access which vary heavily

a multiverse is the span of a bunch of universes i thought

That's pretty much what I'm saying

OK
So Versehounds can travel between universes and multiverses, but can only travel to a limited number of dimensions and can't travel between timelines

legit nothing,

a universe is a timeline and so is a dimension.

Read my comment

A universe is an infinitely bundled together large group of uncountable stars,
A dimension is a plane of existence whereby many universes can be found,
And a timeline is a line of time, showing both what happened in the present and past, but never the future, at least, not to human eyes.