Meta = 1. higher or beyond 2. of change
Data = 1. facts or information used in deciding or discussing something 2. information prepared for or stored by a computer

Meta Data = information that describes the actual data.

Or put differently (added about a good year later):

Meta is a prefix that in most information technology usages means "an underlying definition or description." Thus, metadata is a definition or description of data and metalanguage is a definition or description of language. Meta (pronounced MEH-tah in the US and MEE-tah in the UK) derives from Greek, meaning "among, with, after, change." Whereas in some English words the prefix indicates "change" (for example, metamorphosis), in others, including those related to data and information, the prefix carries the meaning of "more comprehensive or fundamental."