A Facebook (Meta) pixel is like a tiny tracker that helps Facebook (Meta) understand what people do on your website. It's a piece of code you put on your site, and it quietly watches when someone visits or takes action, like buying something. This pixel then sends this information back to Facebook (Meta), so you can see how your ads are working and who's interested in your stuff. In simple words, it helps you learn about your website visitors and make your ads better at reaching the right people.
Check out the pixel creation guide here.
If you already have a pixel but can't find it, please refer to this guide.