Skirmish is a singleplayer gamemode that emulates multiplayer by replacing all teams except black with AI. After starting a game of Skirmish by selecting the icon in the middle at the bottom of the game menu, you will become black's leader and every other team will be AI. The goal is the same as multiplayer, to be the last colony standing. The gamemode uses a modified multiplayer lobby, so you can see which teams are dead by pressing Tab. It is also worth noting that AI colonies in Skirmish don't use the queen menu, meaning that they will need less food than the player to survive, a neat balance considering how utterly stupid they are.