What are Whitelists and Blacklists?
Let's start with Blacklist (you should too). They are a list of ether sites or supplies that you exclude from your campaign. Whitelists are the opposite, a list of ether sites or supplies that you want to exclusively get traffic from.
If you are confused about Site ID and supply go to the Site & Supply ID help doc.
Because not all sources are the same, you will probably find that some work better with your campaigns than others. Creating a blacklist is a good way to continuously refine the traffic to your campaigns. To implement whitelists and blacklist head to the Create Campaigns page and find it under targeting.
Tips & Recommendations:
Always keep track of the sources that continuously underperform the rest.
It is hard to no after how long to wait before blacklisting a source, but we recommend a minimum of 1-3 days or until you have spent around 2-3 times the payout of your offer. We also recommend that you use blacklist before whitelist because whitelist will greatly reduce the amount of supply to your campaigns.
Be sure to create and use different black/whitelists if you change the verticals of your offers.
Checkout How to Optimize for any further help.