If there’s one tool ‘remarketeers’ need to make themselves familiar with, it’s Frequency Capping.
“When you turn on frequency capping for a campaign, you set a limit for the number of impressions you will allow an individual user to have per day, per week or per month. You also choose whether the limit applies to each ad, ad group or campaign.”
Simply put, it’s limiting the times you show a remarketed ad to a user. Because, if you’re anything like me, the more I’m shown an ad (especially if it’s not relevant) the less likely I am to click it.
Read the official Google Adwords help about Remaketing Frequency Capping