If you haven’t tweaked your Google Adwords placements, there’s a good chance your ads are getting tons of impressions on sites that will likely not lead to a click, for instance, GMail.
To fix this, login in to your Adwords account.
Click on a campaign.
Click on Display Network tab.
Click on Placements tabs.
It should look something like this.
Look through the list of sites your ads are running on. Look for the ones that are generating a ton of impressions, but very few clicks. Look for sites that are irrelevant to your ad campaign. Click the checkbox for sites you want to exclude.
Rather than excluding from the Ad Group, I recommend scrolling to the top of the list, click the Edit button and select exclude the selected sites from the entire campaign.