1. Literally, to pull in the reins of a bridled animal, especially a horse, to slow it down or bring it to a stop. Mary drew in the reins as she approached town, bringing her horse to a saunter.

2. By extension, to slow something down or bring it to a stop. During the economic boom, our company was making as many risky investments as we could find. Luckily, our CEO had the sense to draw in the reins on such recklessness before the economy crashed.