Manna

Manna, simply put, is energy. In modern layman's terms, specifically Star Wars jargon, manna is The Force. Manna is the energy that surrounds everything.

Energy is needed to cast a spell. If the spellcaster is gifted with an unlimited supply of manic energy (and science actually calls that a disorder), then he/she can cast a spell without having to tap manna. But for regular humans like us, attempting to cast a spell using only the energy in your body will drain you. Which is why we need to tap an external energy source.

Different places have different types of manna because they produce different energies. To illustrate: A forest clearing, surrounded by all the huge trees, blades of grass, mushrooms, insects, and birds, produces a different energy, and consequently has a different type of manna from, let's say, a collegiate basketball game during the finals. Spells can actually be cast in both these places, but depending on your spell, you have to choose which energy source you will tap. If your spell needs more---aggression---then cast it in the basketball game. If your spell requires more tranquility, cast it in the forest, or on the seashore. It all really depends on the type of spell and its requirements.


*pic from forums.tabcrawler.com