Spindle, how does it work?

Hello everyone i have a lot of questions about the spindle but let's make it simple, what it is? How do i set up the spindle in grbl? I'm using CNC shield (the red one idk the name) and what kind of motor should i use? I actually have a chicago eléctric 110v dremel can i use it? And i think that's all any sugestion would be great! thanks for read