Q: What are masterwork rolls?
A: Most legendary weapons are capable of being Masterworked. The masterwork system is a way for you to upgrade your favorite weapons to their maximum potential.
Q: What does it take to upgrade a weapon to a masterwork?
A: Upgrading your weapons, in broad terms, takes a small amount of glimmer, legendary shards, and, most importantly, Enhancement Cores.
Depending on what Tier your weapon is at, the requirements to raise it to the next tier increase the closer the weapon is to being fully masterworked (Tier 10).
Q: What benefits do I get for masterworking a weapon?
A: Each tier that you increase your weapon will grant a small buff to a single stat, chosen randomly when the item drops.
Beyond that, at Tier 5 you gain access to a kill tracker. At Tier 10, your multi-kills will generate orbs of light.
Q: So what are these "masterwork roll" stats?
A: These are the possible stats that upgrading your weapon will grant a bonus to. Depending on what tier you are at, you can grant anywhere from 1-10 additional points in any of the stats listed to your weapon.
Q: Any catch with this?
A: The Bungie API currently says that all weapons can potentially roll with all stats as their masterwork stat. This obviously isn't accurate, as Blast Radius isn't applicable to, say, Auto Rifles.
The list you see on this page is trimmed to only show stats that actually appear on the weapon. However, it is possible that Bungie has additional logic behind the scenes that further filters these
possibilities to, for example, prevent certain items from being able to have an Impact masterwork. Until Bungie modifies the API files to 100% accurately display which masterwork stats are possible on
each item, take what you see here with a pinch of salt.
In the Crucible, it's more useful on Special and Heavy weapons, as the scarcity of these ammo types means you'll rarely even have a single full magazine for any given Special/Heavy weapon, much less multiple, so the perk is effectively active more often. This is what made Main Ingredient so monstrous, and it's still effective at this today. Also, Aggressive Scout Rifles (currently just Long Arm) can deliberately only reload up to half the magazine due to their interruptible gate-loading reload mechanics, letting you maintain the bonus continuously. Shotguns would also be able to do this, if any Shotgun could get Under Pressure, but none can currently. I think.
Failing that, it's an okay perk, I guess. Mostly for PvP, for tense one-on-one's, to give you a little accuracy edge to hit more shots to kill the other guy a little easier before he kills you. Think of it like Eye of the Storm in that regard - for those desperate moments where you have to hold down the trigger on one guy instead of being able to kill him right away with minimal damage - albeit based on your mag', not your health. By these merits, it's okay in PvE too, for, say, when you really need to keep your gun on-target while DPS'sing down a boss while it's firing on you, but I don't need to tell you that's pretty specific, even if it's a very real situation we've all experienced.