If you would like to have a block only be visible on certain devices, for example mobile only. You can select what devices each block will be shown on. The devices are split up as follows -
After selecting a block you will see the devices setting on the top menu under the block tab. When a device is selected it will be highlighted and have a white underline. For example the block in the above picture will only be shown on mobile devices. You can turn each one on and off until you have the desired layout.
It is important to check the layout of your pages on all devices as some content may not be suitable for all three.