1. adjective In absolute terms, descriptive of a persona which is well on the way to becoming a wiz. Mages are all high level, warlocks probably are, necros possibly are, and sorcs probably aren't.
2. adjective In relative terms, closer to wiz than the rest of the group under discussion. In a reset full of novices, a sorc is high level. In this context, the term is often used in comparative or superlative form, ie. higher level or highest level. Sometimes you even see things like "If there are two mages, I always attack the one that's highest level".
3. adjective In programming terms, far from the machine. C is (supposed to be) a high-level language.

For all the above, the term may be either hyphenated or unhyphenated, although it's usually hyphenated only when directly before a noun or adjective. See level, and the equivalent entries under low level.