index.liquid template renders the homepage of the shop. The template is loaded when you go to the root ("/") of the shop.
Accessing the attributes of a specific object
index.liquid template, you may want to show the contents of a specific object. For example, you may want to showcase the Frontpage collection on the homepage. This is done by passing the handle of a specific object or through a theme setting variable. See Accessing attributes via the handle for more details.