What does rooted phone means?

Basically, rooting your mobile device means to get access to the root of your phone`s operating system and change some stuff there. The phone does not come rooted from factory because the setting that you can change there can brake the software and even hardware of your phone/tablet that`s why

IF YOU ROOT YOUR PHONE YOU WILL LOSE WARRANTY.

The main reason that people root their phones is to run custom ROM`s (Custom operating Systems) or special applications, that usually makes the phones slower, except the memory cleaning tool.

The rooting process is different from device to device, all of this methods are unique, as they require special files, but there are some softwares that can root multiple devices just by connecting them to your computer.
To root your phone you need a microUSB cable and some knowledge, you can brick your phone if you do not root it the right way.