In StarCraft resource refers to natural substance that is collected by workers and required to build units and structures. There are two kinds of resources: minerals and vespene gas.
Minerals (referred to as ore in triggers) is a primary resource type.
Minerals can be harvested from Mineral Fields using worker units. Worker units approach mineral fields and begin to mine them; after a short delay, they carry small Mineral Chunks back to the nearest mineral-accepting building, and their owner gains 8 minerals. Only one worker may mine a mineral field at a time. As mineral fields are mined, they visibly become smaller; when they are used up, they disappear completely.
Bases tend to come with several mineral fields, which may all be mined concurrently by multiple worker units. Because of this, minerals are a very commonly-used resource; nearly every unit, upgrade, and tech in the game costs some amount of minerals. In maps created with third-party map editors, multiple Mineral Fields may be stacked in the same location, giving the illusion that one Mineral Field can be mined by multiple workers at the same time.
Triggers may use the Set Resources action to manipulate the amount of minerals a player possesses. The Accumulate condition can check how many minerals a player has.
Vespene gas is a secondary resource type.
Vespene gas is harvested from special buildings constructed over Vespene Geysers. Worker units enter the structures, and after a short delay, they exit and carry small packages of gas to the nearest resource-accepting building. The buildings vary in name and appearance from race to race -- Terrans use Refineries, the Protoss use Assimilators, and the Zerg use Extractors -- but they are identical in function.
When Vespene Geysers are fully mined, they are said to be depleted. Depleted geysers can still be mined, but workers will carry less gas back when they exit.
Vespene gas is generally used to construct medium-to-high-tier units, such as Terran Firebats and Protoss Carriers. Bases generally come equipped with fewer Vespene Geysers than Mineral Fields, meaning that gas is harvested at a slower rate than minerals; this is compensated for by the fact that fewer units require Vespene gas, and the amount needed is generally smaller than such units' cost in minerals.
In addition to being collected by workers, Vespene gas can also be given (and taken away) by triggers using the Set Resources action (where it is simply referred to as "gas"). The Accumulate condition can be used to check how much Vespene gas a player has.