A defining concept in strategy games, a build queue allows units to be built in sequence. The player can simply queue several units in a production structure and forget about it. The production structure will build the units in the queue.
In some real time strategy games like StarCraft, units in the build queue are charged immediately when placed in the queue. This makes queueing inadvisable because the player is spending money for no immediate gain. Ideally, build queues should be empty in these kinds of RTS's until the current unit is about to be finished. The money should be spent on more production facilities or to tech up to higher tier units. Other RTS's like Command and Conquer however will not charge the player until the unit is actually being produced, making it more acceptable to use the build queue. However building more production facilities is still advised to produce units faster.
Furthermore, some RTS's have a limit to the build queue. StarCraft for example only allows four/five units to be queued up at once. Command and Conquer on the other hand has no limit to the queue.