Since the terminology is new to you, an elaboration on [MENTION=13460]Homebrew01[/MENTION] 's explanation. The mechanisms that actually shift the chain from one cog to the next are called derailleurs (the French are literal-minded, they derail the chain). The front derailleur is the small sort of rectangular cage that hovers above the chain. You'll see that as you work the shifter that cage moves right and left - it's just a brute force mechanism that pushes the chain off one chainring and onto the other.

And as [MENTION=13460]Homebrew01[/MENTION] says, sometimes the chain is at an angle (for example if you're in the big chainring at the front and the big sprocket at te back - this is called cross-chaining) and rubs against the front derailleur, which needs to be shifted slightly out of the way, or "trimmed", to stop the clattering.

Sorry if this is overdoing the explanations, but nobody is born knowing this stuff....