Smartlink is a feature that automatically chooses the best performing offer based on incoming parameters of your traffic (country, browser lannguage, ip etc). So instead of creating several offer links in different campaigns, you get a single link for your traffic.
To get a smartlink, you just need to click Start on the vertical you need.
As a result, you will get a smartlink URL which you can use at your traffic source.
To make changes to the campaign, click Edit in the appeared window and click Save.