A conversion pixel will help you understand if your ads are generating leads.
A conversion is an action you want the user to make while visiting your website.
This could be filling in a form, watching a video, or indeed making a purchase. To track if users are doing the action you want them to do, you should use a conversion code. This is a code (unique for every campaign you will do) that will check if every single user has performed the action directly on your website or via another one.
You may have heard about Tracking Pixels too, but learn the difference: