The Pixel is a small piece of code that you should place on your website to allow conversion tracking.
When someone visits your website and takes any action, from viewing the page to buying something, the Pixel is triggered and reports this action. You will be able to better analyse the performance of your campaigns as well as reach this customer again through Custom Audiences.
More conversions means a better algorithm for Facebook and Dexter, therefore, with time, your ads will be delivered to people who are more likely to take certain actions.