"Weapon slots" refers to the designated spaces or compartments in a game or equipment where you can equip or carry different types of weapons. Full definition
The combat is focused mostly around the use of class skills, as four skills can be assigned to both main / secondary weapon slots. (ztgd.com)
Your first weapon slot are kinetic weapons that obviously deal no void, arc or solar damage but your second slot, now called energy weapons do. (thatvideogameblog.com)
The final weapon slot is occupied by a mine or rocket launcher, which does heavy damage and has limited ammo. (mmogames.com)