Display Density Settings
Winter 19 gives user ability to adjust the display density settings in Lightning Experience. Administrators can select either of them as default and all the users will have the same display density. Users, still have the ability to override this from their profile menu.
Compact setting has labels to the left of fields and less space between page elements and as per SalesForce, it can fit 30% more fields on the same page.
Comfy, on the other hand, is the same display setting that we currently see in the lightning experience. It has labels on the top of fields and more space between page elements.