Not cheating by using Wikipedia, but here goes:

The “Universe” comprises everything that exists. You, me, the cat, your mom, planet earth, the sun, the moon, the stars in the sky, the galaxies, and even everything else that exists that we can’t see.

The “Observable Universe” extends only within our light cone: that is, everything our most powerful telescopes can see, including more advanced telescopes humans will develop in the future. Because light takes time to travel to us, and because space itself is expanding, there are undoubtedly galaxies beyond what we can currently see that we will never see. In fact, the farthest galaxies we can see at the moment will eventually wink out as the (relative) rate of expanding space outruns the speed of light. Those galaxies will cease to exist within the Observable Universe while they continue to exist in the Universe.

I’ll let someone with more wrinkles on their brain dumb down theories spacetime and dark matter and 2D holographic space.