Liquid is a template language created by Shopify. It's available as an
open source project on GitHub, and is used by many different software
projects and companies.
A template language allows you to create a single template to host static content, and dynamically insert information depending on where the template is rendered. For example, you can create a product template that hosts all of your standard product attributes, such as the product image, title, and price. That template can then dynamically render those attributes with the appropriate content, depending on the current product being viewed.
The variation of Liquid in this reference extends the open-source
version of Liquid for use with Shopify themes. It includes tags,
filters, and objects that can be used to render objects specific to
Shopify stores and storefront functionality.