Promotions are used to automatically promote and negate search terms from various ad groups to other ad groups. Promotions are generally used in conjunction with a 3-stage campaign structure - promoting search terms from seeder campaigns, to testing campaigns, and finally to performance campaigns.
At a high level, this allows you to automatically weed out poor performing search terms, while pulling out the high performers to separate campaigns for finer bid control (generally to increase the bids and drive more sales). This combination of pruning poor performing search terms while increase bids on high performing search terms is one of the keys to driving more sales at a lower ACoS.
Promotions are checked once per day at approximately 5:00am UTC. This translate to: