Whitelisting and blacklisting are methods to include (whitelist) or exclude (blacklist) specific websites, apps, advertisers, or ad networks.
An advertiser who desires not to serve advertising on a specific website/app – or even through an entire ad network, can blacklist those. This means their advertising is eligible to serve everywhere except what was blacklisted.
Alternatively, an advertiser who only wishes to serve on specific websites/apps/networks can whitelist those, which means their advertising would only serve on what was whitelisted and nowhere else. Whitelisting too restrictive can result in under delivering.
Similarly, content publishers themselves can block or allow specific advertisers or networks.