The Wait command causes a unit to stop processing of further queued orders until the wait command is removed.
By default, the wait command is inserted in the front of the command queue, while preserving the rest of the command queue. The wait command can be queued after existing commands with the shift key. Further orders can be queued as normal after the wait command, but they will not be executed until the wait command is removed (by reissuing the wait command).
Wait command variants
The normal wait command causes a unit to wait until the wait command is removed; the wait command is not removed automatically.
There are also some special forms of the wait command that remove themselves automatically when certain conditions are met. They are not bound to hotkeys by default, but can be found in the game menu and then bound to hotkeys.
Gather wait causes a group of units to wait until all units in the group have reached the gather wait command. The gather wait command is automatically inserted at the end of the command queue (without needing to hold shift while issuing), unlike the other forms of the wait command.
The death wait command causes a unit to wait until the selected unit dies. Upon activating the command, the player is prompted to click on a unit (friendly or enemy) to wait for. It can be queued after other commands by holding shift while clicking on the unit to wait for (otherwise it is inserted at the front of the command queue).
The squad wait command can only be applied to factories. Upon activating the command, the player is prompted with a slider bar to select the squad size to wait for. Hold shift while selecting the squad size to queue the wait command. Units created by the factory will then wait until a group of the selected squad size is produced before executing further commands.
Note that squad wait is only useful if the squad wait command is itself queued (e.g. after an initial move command out of the factory). Without queuing the squad wait command, it is applied to the factory itself, which will stop the factory from producing anything (if you accidentally do this, issue a normal wait command to your factory to cancel the wait).
The timer wait command is automatically removed after some time. Upon activating the command, the player is prompted with a slider bar to select the time (in seconds) to wait for. Hold shift while selecting the wait time to queue the wait command; otherwise, the command in inserted at the front of the queue.