The pOS documentation covers Liquid comprehensibly and includes Liquid functionality specific to the pOS Platform and Siteglide.
Siteglide runs on platformOS and we use their implementation of Liquid which adds a large range of useful functionality.
The best place to start learning is their Introduction to Liquid documentation, which will explain the different types of Liquid syntax. From there, you can find more helpful documentation on the Liquid topic you are interested in.
We're also writing tutorials on Liquid more specific to Siteglide. You can find these under the "Liquid" collection on the left-hand side.