What does the collection column mean?

The collection column allows you to choose if you want your pixel to fire for all your store, or fire only for products in a certain collection. If you choose "All - Master / Backup Pixel" for your pixel, then it will fire for all your store. Otherwise if you choose a specific collection, your pixel will fire only for products in that collection, and this will allow to segment your data by niche, and optimize your campaigns more efficiently