Which ad rotation setting shows all ads in an ad group, even those with a lower clickthrough or conversion rate?
Optimize for clicks
Optimize for conversions
The rotate evenly setting delivers your ads more evenly into the ad auction. In general, rotating evenly allows ads with lower clickthrough rates and conversion rates to show more often, so this option could result in a lower average position or fewer clicks and conversions.
Read more here: https://support.google.com/adwords/answer/112876?hl=en