Pacing is how we ensure we spend your budget evenly over the schedule of your ad set. It's also a mechanism for helping us meet the cost goals of your bid strategy. It may be useful to think of it as the intersection of your budget and your bid strategy.
"Standard Delivery" spreads the delivery of the ads throughout the timeframe of the campaign and spends continuously and evenly.
"Accelerated Delivery" uses full bid amount to boost the spend and help with winning more auctions earlier. By using the Accelerated option we may end up with a higher cost per optimization event but in competitive times of the year like Christmas, Easter, Black Friday etc., to spend more of the budget and to win more auctions this option can be tried.
For more information on Pacing, you can go to this link.