If I would only know what API does mean...
Probably something with mods ?
Application Program Interface.
Basically, its a standardized set of programming calls that allow modders to interface with the game in a controlled and uniform way.
Think of it sort of along the lines of instead of a modder having to go in and directly change a stored value for something in order to make a mod, they would use the API to "ask" the interface to make the change for them. The modder wouldn't have to know where in the code the spot they were changing was, or how many instances of it were in the code that needed changing, the API would handle all of that for them.
For a really simple version, in programming we consider it good practice to declare global constants at the start of the program, and then refer back to that.
So lets say we wanted to make cannons do a default 2 damage per shot, we could simply do "= 2 + X" for all the different combinations of weapons, but that would mean if we ever wanted to change it later we'd have to stop and search through all of the code for every single instance of "2" in order to find them all. Or we could set a global constant at the start of "CannonDamage = 2" and then use "= CannonDamage + X" everywhere. Then if we ever wanted to change the base cannon damage, we only have to change that first declaration of "CannonDamage = 2" to "CannonDamage = 3" and everything else in the code would update around that for us.
Thats kind of what an API is. Its a wrapper that says "Okay, you can't have access to the raw code, that would be silly. You might break something. But here, here's where you can say CannonDamage.SetValue 2 and then I'll go find where that is in the code and update it for you." It lets you have the ability to make changes, but only in approved ways and in approved areas.