Dimensions are planes of existence that scientists have identified. Travel between dimensions can be tricksy, but it is possible. They are related to the concept of Alternate Timelines, and many use the terms interchangeably, but while alternate realities connote a "parallel" plane of existence where maybe one thing is different (everyone wears hats or some bullshit), dimensions can also include higher states of consciousness, or even lower or nonexistence itself.
This is the realm of nonexistence. It doesn't exist. You can't go there, and even if you did, you would cease to exist, and probably wouldn't have ever existed at all. This is where the Corrector and other various Cosmic Overseers send mistakes that would otherwise break the laws of physics.
This is where pointilism happens. It is the realm of the microverse, of the lowest possible subatomic particles, and thus is not conceivable to feeble human minds. You can go there, but you will probably go insane or at least end up catatonically dumb. That is why this dimension correlates to the consciousness of rocks.
This is Flatland, where space only moves in two directions, and any higher-dimensional beings are perceived as God-like and mysterious. Bacterium, amoebas and viruses inhabit this realm, and their level of consciousness is commensurate with two-dimensional thinking.
This is where you and I, and all the plants and animals live. This is the realm of known space, and anything higher confounds us to our horrified core. We know just enough to know that we know nothing, and stand aghast at a ever-expansive and enigmatic reality just beyond the grasp of our little intellects, and thus occupy ourselves with material goods like food, "bling" and sex.
Most of the other "dimensions", "realms", or alternate realities that you can travel two are actually sub-dimensions of the Third.
This is the Dimension of time, and many cosmic beings, greys, demi-Gods and ascended warriors live here. The Dead go here to remain deathless, and those within the fourth dimension realize that time is an abstraction, as easily traversed and meaningless as space is to third-dimension dwellers. This is the final phase up that those in the third can even partially wrap their head around (and even then), and so fourth-dimensional creatures like Tralfamadorians are constantly having to roll their eyes and talk down to mortals like they were dumb babies.
Fifth DimensionEditAnything above the Fourth Dimension is a horrifying exercise in insanity, and your skin will probably melt if you were ever to behold even the tiniest fraction of a glimpse that your brainmeats could parse in their pathetic limitations.
There are many types of dimensions, and all dimensions are beautiful. Do not shame dimensions. Beings travel different dimensions via dimension door rips, astral projection, wormholes and timefuckery. There are entire realms ruled by a single Platonic ideal, such as the popcorn dimension, or the Dark dimension. Most humans will never see these, and no Norms have ever accomplished this feat (it would sort of make them not Norms anymore to be honest).
Each of these stupid alternate timelines or realities can and will interact at some point (infinity demands it) and results in cataclysmic collapses of one or multiple dimensions into each other, such as DC vs Marvel . Eventually, they will all be destroyed and rebirthed, becoming one and the infinite many.