The whole bolt carrier group. Once you get it broken in good read Robert whitley on how to lubricate an AR properly
http://www.6mmar.com/site/mobile?url=http://www.6mmar.com/Info_Page.html#2729
How To Lube Your AR-15 (Very Important)!
People speak of running AR-15's "wet" but this is not the way to go at all. An AR has gas blow back and gas with particulate matter gets back into the inside of the upper receiver, the bolt carrier, etc. Having a wet sloppy mess of oil and grease in there does nothing but provide a trap for a big grungy mess to form and clog things up.
The right type of lubrication needs only be applied where there is metal to metal contact of moving parts - any more is a potential cause for problems. You don't want gobs of oil and grease shooting and squirting all around inside your rifle as it cycles, sending lube into places where it does not belong (i.e. down in the magazines, on the bullets themselves, up inside the chamber, back into the buffer tube assembly).
Would you indiscriminantly hose down the inside of your car's engine compartment and the drive train underneath your car with lube and oil? No!
On lubrication, about 95% of all "gun" lubricants out there are not appropriate for an AR. For the bolt lugs in particular you need a very high viscosity sticky lubricant because the lugs are a "severe duty" application in that they open fast under pressure and get very hard use otherwise. Most greases are no good because they don't stay around. The best lubricant I have found is Phil Wood Tenacious Oil (it's like a sticky 90 wt. gear oil) and you can buy it for $8 or so at your local bicycle shop or on the internet in its own applicator bottle. I have built up a lot of AR's over the years and seen others come back for re-barreling or other work. Some have an enormous amount of bolt wear (and a corresponding dramatic increase in head space) because inadequate lubricant was used. The owners thought they were using a good product, but it was not. A lot of the favorites are no good either (and I am not going to name names).
Some parts not to lubricate at all: 1. The firing pin (don't need anything to impede the pin making a good strike on the primer); and 2. The buffer or the buffer spring assembly (unless you want your rifle to possibly malfunction and short stroke); 3. If you are going to be shooting in very cold weather, keep the lubricant off the lateral riding surfaces on the outside of your bolt carrier or it can slow the carrier down enough so the rifle will short stroke. Above about 20 degrees F, that's not an issue typically.
The areas that always need it: 1. The rear of the bolt lugs (severe duty – must do, but only need a little bit on the back of each lug); 2. The cam pin (another severe duty place, put around the pin just below the head of the pin); 3. The very back of the bolt behind the gas rings (the .250" diameter stem) where it rides in and out of the carrier; and 4. The gas rings; and 5. The lateral riding surfaces on the outside of the bolt carrier, except in very cold weather (like below 20 degrees Fahrenheit) where high viscosity lube can slow down the carrier and possibly cause the rifle to short stroke).
