It's short for Script Kiddy. They just use all the programs and scripts that other people have made rather than writing them themselves.
Slight correction, if that were true most of us would be skids.

Skids (script kiddies) use scripts/programs to "hack", but have no idea how they are doing what they are. For example, using Ophcrack without knowing what rainbowtables are. Even the best of us will use tools to save time, but we actually know what we are doing and how we are doing it.