Default [Q] Root,Unlock,and how to use SIM from another company(Galaxy Nexus)

I'm new to android.So sorry if I ask simple questions , but I don't know what is unlock , root and moreover how I can use SIM from another company in my phone?
Please guide me about this procedure,I mean at first I have to unlock ,then root,or what else.and after these steps could I use different SIM?

Thank you