A remarketing list is a collection of website visitors or app users. When you create a list, you set rules for when visitors or users should be added to the list. You also specify a membership duration for how long you want your visitors or users to stay on the list. Once you’ve created remarketing lists, you’ll need to add them to your campaigns’ ad group targeting so that you can show ads to visitors or users on your lists.
Remarketing lists targeting the Google Display Network must have a minimum of 100 active visitors or users within the last 30 days for your ads to show; lists targeting Google search must have a minimum of 1,000 active visitors or users.