The air gets sort of charged. Not... quite like static electricity, but there's a sort of pressure? [Shrugging, Cloud's gaze goes a little distant as he thinks back on what he's been told.] Principle is similar though, a difference in pressure and energy between the materia and the air around it.

[Looking back at Ahsoka, Cloud doesn't need to be prompted to explain that, just continues, because this, at least, is easy. He knows all of this, even if he hasn't ever gotten to use materia himself, yet.]

Materia's small stones which is what we use to cast magic. They're... solidified pieces of mako, which is a liquid form of energy.