Skirmish is a special game mode in in all of the Command & Conquer games. It's essentially a multiplayer mode with only one human player, against single or multiple AI opponents, it only needs one computer and doesn't require network connections to be playable.
The player is always the "host" and can decide conditions of a skirmish battle via adjusting various options. Including player name, factions, colors, maps, number of AI opponents, AI levels, starting credits, etc. However certain options are not available in particular games.
In earlier games (or earlier versions of said games), progress of a skirmish cannot be saved like campaign missions, but this is corrected later.