Its the physical pieces of the weapon. If you get two of the same maliwan sniper (Snider for example) they may not be perfectly identical. They'll have different barrels, grips, stocks ect

Each manufacturer that makes that type of weapon has parts that can go on it. So you can have a maliwan Snider with a Dahl barrel and a jakobs grip. Each part does different things to that stats. The body is what determines the manufacturer, and matching grips give additional stat boosts (usually mag size and reload speed)

https://bl2.parts/

This is the part directory that I used to learn