In StarCraft II, the maximum 'supply cap' - a number that dictates how many units you can have active at once - is 200, regardless of the race you play. Your headquarters building (command center, nexus, or hatchery) provides a small amount of supply. In addition, each race has a special building or unit that can be built to increase your current supply cap in regular increments. The terran race builds supply depots; the protoss build pylons, and the zerg build overlords. Try to never forget to increase your supply cap by building or morphing these units and structures. Forgetting to get them out on time (aka ‘getting supply capped’) can often put you at a disadvantage. For terran and protoss players, no matter how many buildings or units you create, you cannot exceed 200 supply. If Zerg constructs buildings with their drones, produces units with the supply that was freed up, and then cancels the buildings, then they can have more than 200 supply.
Freeing Up Supply Space
Sometimes, you'll run into a situation where you need to quickly build specific combat units, but you do not have the supply to do so. In desperate situations, kill some workers or the most useless unit you can find to free up supply for units that would better address your current predicament. While this may seem an extreme solution to lack of supply, it can help out in dire situations. Terran players can solve this problem by using the ‘Calldown: Extra Supplies’ ability (available at the orbital command) on an existing supply depot.
Destroying Enemy Supply
If you destroy your enemy’s supply units or structures, they have to generate more supply before they can build more units. Even if you have a bunch of bases, unit-producing buildings, and resources, you cannot build combat units if you lose the supply buildings in your main base.
It's easiest to kill zerg supply units - air units such as vikings can efficiently slay overlords.