Bootable USB is used to install the operating system. To create it you will need to perform several easy steps:
Press the icon on the toolbar.
The following window opens. Choose the type of OS your image file is, pick the USB drive, press Refresh if the needed drive is not on the list, choose the image file you want to burn. Once you've done, press Continue.
All your data will be wiped from the chosen USB drive. Make sure you've saved all the important files before you press Continue.
When the process is finished your USB drive is bootable. You can install the chosen OS from it.