Rooting is the process of giving yourself ultimate system permissions on your phone. Rooting allows you to do many more things that you cannot do without it, such as installing custom ROMs, and running apps that require root permission such as screen recording software and customizing apps that allow you to change almost every aspect of your phone. You can also uninstall apps that come default on your phone (commonly referred to as bloatware). You can even install a completely different OS on your phone such as Ubuntu. There are a few drawbacks, such as, it will void your warranty, and if you do the rooting process correctly, you can brick (ruin it where it is as useless as a brick) your phone.