Minute Physics has undertaken the daunting task of explaining the Theory of Everything. But not surprisingly, it's going to take more than just a couple of minutes. So they've broken it down into smaller chunks, starting with an explanation of matter.
In laymen's terms, matter is "everything that takes up space", but that explanation is flushed out a little more thoroughly through the Pauli Exclusion Principle, which states that particles like electrons, neutrons, and protons can only ever occupy one point in space at a time. As a result, when they come together to form an atom, the results end up occupying physical space.
Got it? Good, because delving into the Theory of Everything is only going to get more complicated from here. [YouTube]