Vision:

Vision is the information a team has about parts of the map and enemy units within its sight range. In Dota 2, it is created by heroes, creeps, buildings, wards, and some abilities, and it is essential for spotting enemies, avoiding ganks, and setting up fights or objectives.

Control of vision often decides when a team can farm safely, place wards, or contest Roshan and towers. Denying enemy vision with sentries, smoke, and map movement is a major part of support play and map control.