Delrin is a metal substitute. It's an acetal resin engineering plastic that is light weight and low friction. That is just a complicated way of saying it's a great material to make internals out of because they don't require oiling and the lighter weight promotes greater efficiency throughout the marker.
These after market bolts are fairly inexpensive, about $20. Tactical Mods sells Delrin bolts for various Ariakon and Armotech markers (Sims, ACP, Zeus, etc.) as well as some others. Alamo Paintball also carries Delrin bolts.
The lightweight bolts and strikers are great especially for markers that use 12gram cartridges. The stock aluminum bolts are heavy, so they cause "bounce-back". When the trigger is pulled the striker hits the valve and opens it releasing the gas. When the striker hits the bumper it bounces back and hits the valve again releasing more gas, but the paintball has already left the barrel. That means almost an entire shot was wasted every time you fire a round.
In some conditions this can also have a slight "blow-by" effect causing the shot to spin and become inaccurate. The lighter weight also means it takes less air to move the bolt so more of the potential energy can go towards propelling the paintball at your target. A Delrin bolt improves air efficiency above all, but also improves accuracy and distance for these reasons. Deifnitely a great buy for any marker and it's only around $20.
Edit: Here's a post by Sgt. Slaughter from the Ariakon forums further explaining Delrin bolts and the material.