RON:
"essential vs. nonessential modifier".
NOTE: i've never seen this issue directly tested, so it's not important for you to be able to distinguish between the two -- the only thing that's important is for you to realize that both are ok, under certain circumstances

an "essential" modifier (which is NOT set off by commas) is a modifier that actually narrows or specifies the noun/action to which it's attached.
for instance:
our top student whose score fell below 50 received a consolation prize.
--> this doesn't refer to the top student among all of our students; this only refers to the top student among those whose scores fell below 50 points. therefore, we need the essential modifier (no commas) to narrow "our students" to "students whose score fell below 50".

a "nonessential" modifier (which IS set off by commas) is a modifier that does not narrow or specify the nouns/action to any greater degree than does the rest of the sentence; it merely provides more information about that noun/action.
for instance:
our top student, whose score fell below 50, received a consolation prize.
--> here, we are actually talking about our top student. the meaning of the sentence is that all of our students' scores were below 50, but at least our top student received a consolation prize.