Meta is from the Greek word μετα, a preposition that originally meant "among" and later took on other meanings like "with" and "after". It's related to words like the English 'amid' and German 'mit'. In Latin it was often translated by trans.

Often meta has a reflexive meaning, as something self-referential. Metacommunication (communication about communication), metanode (a node about nodes).

Sometimes meta implies a change in state: metathesis (transposition of sounds), metamorphosis (change of shape).

Sometimes meta implies a deeper meaning: metaphor (figure of speech), metaphysical (supernatural, ontological).

Of course, the meaning of 'meta' is anything but nailed down; half these examples don't work diachronically ('metaphor', for example, could easily be nothing but a calque of 'transfer' or 'carryover').

In emacs Meta is usually the Alt key.