Dual booting means having more than one operating system installed at the same time. Usually with Windows, as long as you install the later version of Windows after the older version, it should set up a menu on boot where you choose which version to load (possibly with a timer so it will load the default option if no selection is made in 30 seconds).
A backup would be a good idea before installing a new version of Windows. If possible, the best way to install it would be to a new hard drive. It may also be possible to repartition your existing hard drive to make room for the new install, if you have enough free drive space, or you can backup everything, wipe the existing install and do a clean install of Windows 10 (possibly the cleanest / easiest way to upgrade, but the most final in terms of not being able to easily revert back to the XP install, and no way to recover anything missed in the backup).
