Floors are simple a way of grouping spaces. For example, most users choose to group outdoor spaces into an “exterior” floor. Similarly, some users working with an additional building will rename floors to distinguish the different structures (e.g. "House - Main Floor", "Guest House - Main Floor").
To view the floors in your project while on desktop, click on Spaces (boxed in black in the picture below).
On mobile, click on Spaces at the bottom of the app (boxed in red in the image below).
Further Reading: