Sources are nothing but publisher sites & search engines associated with us. This is where you’re Ads will appear and thus generate clicks. So it is very essential to add Sources to your Campaigns.
Upon assessing the performance of Sources that you are using based on the Impressions, Clicks and Click-through Rate (CTR) they generate; you can add these Sources to Favorites or Turn On or Turn Off the same. You also have the flexibility to define the Delivery% (rate at which your Ads should appear) – per Source.