The Astral Plane is timeless, almost featureless, and is home to everything non-physical or beyond mortality. It's not true to think of the Inner Planes as inside the Material Plane and the Ethereal as the goo holding it all together, but it helps. It is otherwise a foggy realm that connects the Inner Planes and the Material Plane. The Ethereal Plane can look upon the closest plane. Near the outer areas of the Astral Plane, itself an eyeboggling concept due to its infinite size, are the realms of the abstract where the gods live and souls travel upon death, the Outer Planes. Unlike the Ethereal Plane, travel in the Astral Plane is tied directly to thought, and it is believed that intelligent creatures travel to portions of the Astral Plane as they sleep. Everything in existence (except, due to the weirdness of cosmology, the Inner Planes) is accessible from the Astral Plane.
There are also psychic winds that howl across the starry expanse that can shove you to parts unknown and drive you mad.
One can reach just about every plane of existence from here. The Astral Plane, on the other hand, is a timeless void. The Astral Plane, much like the Ethereal Plane, is another place where planar travelers can have a hub to connect to other planes except on a grander scale. A mirror image of the Prime Material serves as the Feywild, and the shadow the Prime Material Plane makes is the Plane of Shadow. Think of the various planes of matter and energy as a sphere, with the Prime Material as the globes surface and the different Elemental Planes bobbing about inside it. While in the Ethereal, you can perceive the Material Plane if you're close enough to it, a region known as the Border Ethereal. Anubis in this instance rules over a bleak area of the Astral plane. The Ethereal Plane is what connects the Prime Material Plane, where all elements are found, to the Elemental Planes, which serve as the sources of those elements. These are 5e spells, and Im assuming they Diseases have been seen as a terrible. Since you're asking about 5th specifically, we'll avoid talking about earlier editions. In 5th Edition, the primary source of information about both planes is the DMG.