The Material Plane is the physical universe of which we are a part of, and the primary setting for campaigns in the Tetraverse. Elements from the other planes can often enter into the Material Plane in one way or the other, and it can indeed act as a hub for travel between the other planes (Although interplanar travel is extremely rare and not often without consequence).
The Material Plane was the fourth and final plane that was formed from the Great Void. It was designed and created by Su'Klorel, one of the Ten Voidgods. The plane is meant to serve as a "place of growth" where many races can evolve and possibly ascend to the Astral Plane. The nature of the plane suggests that Su'Klorel designed it to be a sort-of testing ground - a place to see which traits and cultures are the most successful. He situated it between both the Astral Plane and the Netherworld, allowing equal influence by forces of either plane.
Su'Klorel designed the plane to be a very physical one. Although magic exists in certain parts of the plane (Namely those "closest" to either the Netherworld or the Astral Plane), the plane's workings are purely physical. Some magicks can alter the physical nature of the plane, but they still depend on that very nature to be effective themselves. It is unlike the other planes in that what a being perceives exists independent of their mind; the plane exists and behaves according to Su'Klorel's design, not the design of the beings that inhabit it. This makes its nature opposite that of the Astral Plane and the Netherworld.
The Material Plane is made up of galaxies: clusters of stars, planets, and other cosmic bodies and drift. It's the most chaotic of the planes, being the newest of them all.
A vast array of physical laws govern the workings in the Material Plane. The effects of spacetime and gravity on all bodies in the plane are immense, often causing many of those bodies to exist and move in patterns that are predictable using scientific methods.
Most beings that comprise the Material Plane exist on planets or Cosmic Land. Similarly, most interplanar travel points exist in these locations, as the chaotic nature of outer space may cause harm to beings who enter it without preparation.
Various races were "seeded" throughout the plane by Su'Klorel. The Ancient Ones (Who called themselves the Vanir) were seeded first. They resembled humans to a large degree, but were gifted with wings. Of the original races created by Su'Klorel, only the Vanir survived long enough to build a civilization. However, the Vanir eventually ascended to the Astral Plane, but not before seeding some worlds with beings who would eventually become modern humans.
The Almarians are the principle race in the Material Plane, being the most advanced civilization that currently exists within it (Though they were seeded long after the Vanir). However, even the Almarians do not occupy all of the vast regions of the Material Plane, which is home to multitudes of different species. Humans are commonly found throughout the plane, being one of the most efficient evolutionary forms and being the second evolution of the Vanir. Elves also existed, before a galactic engineered plague created by Asmodeus wiped them out. Gnomes are also numerous and are currently the only engineered race that exists within the plane; they were created to be the second evolution of the Almarians, possessing the brainpower and aptitude for science while still being naturally gifted with magic.
Hundreds of other races exist throughout the plane, and hundreds more half-breeds.
The Material Plane is essentially the known Universe.