Micro is an abbreviation for Micromanagement. It is related to Macro (Macromangement). Micro refers to the management of precise unit intricateness, such as their position and their exact behavior. Microing a group of units is aimed to optimize their usage and give the player an advantage in the current situation.
Detailed management of units in combat aims to maximize damage given to enemy units and minimize damage to the player's units. In combat a common technique is to let units attack into formations, for example to keep lightly armored shooters behind, protected by more heavily armored units in front of them. Another example is to kite units along enemyies so that they stay out of range while they can shoot at their opponent.
Stop-micro is also a common form of Micro. By giving a (group of) unit(s) the Stop-command they will automatically retarget on their preferred target. For units with AA capability this means that they will auto-target Air units first.
Ore Trucks are typically micro'd as well, as their standard behavior to seek ore is inefficient. Players micro their ore Trucks to optimize the traveling distance or to clear an area of ore so that players can build there.
On the other hand macroing involves more long-term and less unit-specific strategic planning. Microing tactics usually involve quickly reacting and issuing commands (such as the Crushing tactic), whilst Macroing tactics usually involve long term planning (Such as the expansion strategy/tactic).
Building placement is usually considered Macroing.