A universal pixel is a small snippet of code that should be installed in your theme.liquid file
How to Install the Pixel:
4) Select 'theme.liquid' from the layout folder.
5) Scroll down to the bottom of the file and place your pixel just before the closing body tag.
6) Click 'Save' at the top of the screen to save the file. That's it.
1) In order to add the purchase event to the checkout confirmation page you'll need to navigate to Settings > Checkout
2) You'll need to find Additional Scripts in the Order processing section.
3) Place the conversion pixel in Additional Scripts and save.