Even is a smartx pacing setting where the ad server spreads the delivery of impressions evenly across the whole timespan of the campaign.
After 10% of the time, 10% of the impression goal should have been reached. After 20% of the time, 20% of the goal should have been reached, and so on.
Even pacing might be selected if an advertiser is looking to maintain a consistent level of engagement (clicks, conversions, sales) across the campaign.
See also, Pacing.