The Canonical snippet places a proper canonical tag in the head section of pages. It executes only for Symlinks to prevent search engine penalties for duplicate content.
Go to System | Package Management on the main menu in the MODX Manager and click on the "Download Extras" button. That will take you to the Revolution Repository (AKA Web Transport Facility). Put Canonical in the search box and press Enter. Click on the "Download" button, and once the package is downloaded, click on the "Back to Package Manager" button. That should bring you back to your Package Management grid. Click on the "Install" button next to Canonical in the grid. The Canonical package should now be installed.
Place the following tag in the <head> section of all templates:
[[!Canonical]]