Remarketing is a targeting option that can be set for conversion actions tracked by Dable scripts.Several scenarios can come out based on the conversion scripts installed on the landing page.
To use Remarketing, you should have Dable conversion script installed on your landing page.
Conversion script installation guide can be found on the Conversion Script menu.
If you narrow your target too much when there is not enough target user (e.g. campaign has just started), reach and budget spending may decrease significantly.