By default all Frames are set to show and all devices; Desktop, Tablet, and Mobile.
It's often advantageous to show less on Mobile devices, therefore you can simple uncheck the Layouts and Frames that you do not wish to show.
ONE: VIDEO ABOVE
Specific Layouts for Specific Devices
You might wish to present very specific version on Mobile versus Desktop. In which case, you can clone an entire Layout, the make modifications for each Device. Then, in each Layout, simply select the devices where you want it to show.
TWO: VIDEO BELOW
Hide elements on the inside
If you unckeck a device, it will hide everything on the inside of that Frame. Therefore, most beginners, will keep it simple and unhide Full Layouts, like the example Video above. Though, more experienced designers might decide to Hide very specific elements. Such as only show a block of Text on Desktop, and Hide it on Mobile and Tablet.