While editing a line item, you will also be able to choose the delivery or pacing options.

You can select

Evenly Paced: all the targeted impressions will be spread evenly during the day according to a proprietary pacing algorithm. This optimal distribution algorithm takes into consideration the impressions delivered so far against the final goal and takes a decision on when it is more beneficial to deliver the ad. That will ensure your campaign won't be delivered too slow, failing to reach the intended goal, or too fast, consuming all the available impressions ahead of schedule.

Or

As Fast As Possible: the line item will deliver all the targeted impressions as quickly as possible until the goal is reached.