Whitelist for advertisers is a list of websites that will be targeted by their ads. The opposite of this concept is the blacklist.
Publishers also have a whitelist, but for them, the whitelist means the advertisers that the allow to bid on their inventory.
Blacklists are more used in advertising and in publishing. With whitelists you just allow a group. Whit a blacklist you just exclude a group. And as you don’t want to exclude the majority of the players, most of the advertisers and publishers just use the blacklist, unless they have a specific order.