Weapon pick-up is when a player has died, and they leave behind there primary weapon. By aiming at it and being within about a 1 metre radius, press 'E' and the current weapon your holding will then be swapped with te weapon on the ground.
Only primary weapons will be dropped. Therefore you cannot pick up Melee, Specialists weapons/items, Support weapons etc. When picking up a weapon, you do not get that weapon in your inventory, you only can use it until you die or pick up another weapon.