Roottest

A device is called 'rooted' when it has been 'flashed' and the systemuser called root can be used.

Normally phones are not rooted (though there are a few exceptions) and thus you can't use them to their fullest potential. Using root you can remove pre-installed apps, access apps private data, install custom bootloaders etc.

Rooting your phone is easy and can is some cases even be done by apps. But bear in mind that rooting can be dangerous, potentioally bricking (requiring a total wipe to resolve) your phone and that your warrenty could become void.