Can I have more Information about :GetChildren() ?

When I use :GetChildren() , what is it's argument? How can I use this to check for a certain thing within a player? would it basically be using a PlayerAdded event and then use IsA(ClassNameHere) ? I will be looking for a classname not name. Should I be using a Generic for loop such as for i,v in pairs do ? Any information that can be given would be helpful!

I have looked in the huge list within studio and searched some things in Wiki and couldn't find what I wanted.