Supply

Each Unit adds Supply to your total, and if your Supply is full, you won't be able to Produce any more Units.

Overview
Supply is an often used slang term describing the "food" requirements of an army. Technically, Supply is the name of the Terran food counter. Protoss Psi and Zerg Control serve the same purpose as Supply, but most commonly all three will be referred to as Supply. Supply levels may be denoted as x/y, where x is the Supply used, and y is the total available Supply. This can be viewed at the bottom left of the screen, above the Minimap.

Each combat unit or worker you make occupies a certain number of Supply. Open Supply is increased by building specific Structures or Units, up to a maximum of 200 Supply. Certain abilities like Mind Control allow you to bypass this number, which is called the Supply cap. Forgetting to build enough Supply producers often leads to the inability to manufacture more Units, referred to as "Supply capping" or "Supply blocking". Destroying these producers is often employed as a harassment tactic, particularly against Protoss and Zerg.