It 'turns off' Shopify's stylesheets during the 3 step checkout:process (including the mobile styles!)
- Step 1 being: Discounts, Shipping method and Newsletter
- Step 2 being: Order processing screen (spinner)
- Step 3 being: The Thankyou page
Shopify's checkout pages are notoriously tricky to style, especially if you are creating a responsive design.
Shopify does not inlcude the essential viewport meta tag unless checkout.mobile.css.liquid is present in your theme... and even if it is, they lump you with another set of styles to tweak and override.
This reset stylesheet allows you to create a single responsive stylesheet rather than two depending on the environment.
You may be able to get away with not creating the mobile stylesheet (and therefore not using the viewport meta tag), but instead using the CSS viewport declaration. However at the time of writing it was not widely supported.
This reset stylesheet was created by combining all the selectors & properties from Shopify's checkout stylesheet with those from their mobile checkout stylesheet. Then, each property's value was reset. Thereby maintaining the same specificity, hence it is a pretty brutal reset, which includes some !important rules.