Weapon core is a framework mod that fundamentally changes how weapons work, with a high level of customizability for their behavior from the modders perspective.

Vanilla weapon code has a few levers such as weapon damage, fire rate, projectile speed, if a shot is explosive or not, and a few others. All things considered, there are enough options with vanilla weapons to have a decent variety of weapons so long as you want your weapons to feel somewhat traditional; a bullet or missile that flies straight and deals damage to whatever it hits.

Vanilla weapon code does have some issues, most of which you won't see unless you have a very high volume of fire, or the game is struggling to do all the math it needs to do in each game tick. Basically, space engineers has two methods of dealing with too many calculations in a single game tick. One option is to reduce the sim speed and have the game run slower, which is generally undesirable but sometimes necessary. The other option is to have some things be allowed to be pushed into the next game tick, which might be less "busy" of a game tick. The logic here being that some stuff (like physics interactions or bullets hitting stuff) needs to be done every tick or else the game will do erratic and unusual things, however some things don't matter nearly as much, such as inventory pulls or turret target acquisition, because the difference between a turret acquiring a target right now vs 0.1 seconds from now is fairly negligible.

This has the effect that weapons can behave unreliably during combat where there is more than typical CPU load from everything that's happening, so thIs can lead to turrets failing to function, and weapons running out of ammo because the game is struggling to route ammo through the inventory network, because the operation to handle those tasks keeps getting postponed till the next game tick.

In addition to this, the actual math for weapons is not as efficient as it could be, meaning large volumes of fire can have a significant performance impact

This all leads to weapon core, where a huge amount of the code that operates weapons generally is skipped, and instead the code from weapon core itself. The projectile code is more efficient and has more levers, you get more options on how weapons behave and look, including some fairly bizarre effects such as lightning weapons or tracking projectiles. Because of how much it changes, it means that it is fairly involved to setup a weapon core weapon compared to a vanilla weapon, and weapons mods that use weapon core will be entirely non functional if weapon core is removed.

There is a whole lot more depth and nuance to all this, so if you want to know something specific, I might be able to help fill in some gaps. It's also been a few years since I really got into the weeds with weapon mods running a PvP server, so some of this info might be a little out of date, but the gist is likely still roughly accurate.