Taken from Lot 51 Resource:
The first step of modding is to familiarize yourself with the code that make up mods, XML and Python.
XML - This will be the primary technology you'll need to understand to successfully create any form of custom content for the game. XML is used for most Tuning files, which explain how to the game specific object or interaction should function.
Python - A scripting language that powers the simulation code. You will need to understand the rules and syntax to successfully create a Script Mod.
Taken from this forum question
A Tuning Mod is a package file written in XML that changes the default ways in which the game operates. Some popular examples would be no singing in the shower, meals that are group servings default to some other number instead of 8, no (fewer, or more) zombies spawning, more difficult (or easier) homework, etc. Tuning Mods are different from Script and Core Mods because the latter are complied code that run alongside of the game engine whereas Tuning Mods are read once during startup, their changes get applied for that session, and then they are ignored.
<aside> 💡 For an in-depth explanation about the different kinds of mods and how they work in-game, I would HIGHLY recommend reading this blog post from Carl Sims.
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