Minerals are one of the two major resources used by all the three races in the Starcraft game. There are needed as the required cost (often along side vespene gas) to create units, build new buildings and research upgrades in the world of Starcraft. This makes them an essential part of the game as no race can create much of a force with Minerals.
Minerals appear as a light blue crystal on the scenario map. The more densely packed they are the more Minerals are available form that patch. Each patch of Minerals only holds a finite amount of Minerals and eventually they will become depleted. As they diminish the graphic will become less dense and sparser looking.
Your race-gathering unit that is the Terran SCV, the Zerg Drone and the Protoss Drone mines minerals. After a short time period and a small mining sequence the gathering unit will take a small batch of minerals back to the nearest primary building. You can see the small light blue crystals on the sprite as the gathering units transport them about. The primary buildings are the Terran Command Centre, the Zerg Hatchery (or its upgrades the Liar and Hive) and the Protoss Nexus. This means in order to gather resources swiftly it is wise to build a primary building as close top Mineral patches as you can.
The eternal war machine in Starcraft also means your forces will be in constant need of Minerals so it is expedient to gather as much as you can. Each gathering units mines eight Minerals for every trip from the Mineral patch to the primary building. Obviously then the more gathering units you have the faster you will gather Minerals.
In campaign games Mineral resources are often limited and to gain access to more often new areas must be captured and secured and a new base built. They also tend to be quite finite so make sure you make the best use of every thing you create, as wasting resources will often see you failing the mission.
In multiplayer games the amount of Minerals tends to be higher though there are some maps that are not generous in this respect. Even with generous Minerals on a map the amount you use may still mean you need to create further bases to gather resources. Resources your opponents will both be eager to prevent you having and to gain for their own ends.