Layout files are very important for your theme because every other template file is rendered inside the active layout.
There is only ever one active layout on your store at any given time. The active layout changes when your online visitors begin authentication process.
The theme.liquid file is the default active layout when visitors are browsing your online store and/or website. It usually renders the header content, footer content, navigation, and other global variables.
Inside the layouts folder you can add as many layouts as you like. These can be used by the templates by placing the following code at the top of the template file.