Imagine a ship that has 1 turret slot. Industrial transport for example, 1 turret slot on it. You can only install 1 gun, or 1 mining laser on it.
But the ship will show 2 guns, one on top and one underneath. This is just to cover all angles of fire. If there was only 1 gun on top of the ship, you’d have a blind spot if the enemy got under your ship, and CCP didn’t want that. So each gun that you install in the fitting screen will have 2 guns appear on your ship, one on top and one on the bottom, or one on the left and one on the right. Just to cover all angles, basically, no blind spots.
Now, Tristan can install 2 guns in the fitting screen. Cruiser can install 4 guns. Battleship can install 8 guns. You shoot your guns by pressing F1, F2, F3, F4, …, F8. Or clicking the icons. They’ll shoot separately, responding to the timing when you pressed your F1, F2, F3, etc.
In the fitting screen, you can drag all the ammo out of the guns, then drag one gun over the top of the other gun, and they’ll become LINKED. Then put the ammo back in. So now when you go out in space, you just press F1, and all the linked guns fire synchronized. Saves you from having to press F2, F3, etc.
You can only link guns if they are the exact same make and model. All guns, exactly the same. You can only link guns or missiles. You cannot link mining lasers, shields, armor, drones, electronic jamming devices, salvagers, tractor beams, etc., even if they’re the same. Just guns or missiles.
EDIT: Actually, drones, when you’re flying your ship in space, you can right-click each drone and “add to group”. Put them all in a group, name it whatever you want, “light drones” for example. Then order the group to launch, attack, return, etc. That’s similar to linking your guns.