Mineral Line is a term that refers to the space between a group of mineral patches and a gathering facility (e.g. a Hatchery) in which worker units (e.g. Drones) move back and forth, gathering and returning minerals. Mineral Line Saturation refers to how many workers there are in the mineral line in comparison to how many mineral patches there are. Generally a ratio of about 3 workers to 1 mineral patch is ideal, but sometimes less or more workers are used. The mineral line is a very fragile and important part of a game because it dictates the economy of a player. Because of this, mineral lines are often the first thing to be assaulted during a successful attack, and are frequently the target of Harassment.
forgot to build photon cannons for defense and now his probe count and economy is in disarray because he allowed hellions into his mineral line.