Before starting with Fabric modding, it's important to understand some of the key terms and phrases used in future tutorial pages. It's also good to know basic conventions for things such as package structure and modid naming. Knowing these early will help you to understand tutorials better and enable you to ask better questions when needed.

Throughout the documentation, we'll often refer to a Mod ID, or modid in code. Mod ID stands for “Mod Identifier,” and it is a string that should uniquely identify your mod. Mod IDs are commonly associated with identifier namespaces of the same name, and as such, follow the same restrictions. Mod IDs can consist only of lowercase characters a-z, numbers 0-9, and the symbols _-. For example, Minecraft uses the minecraft namespace. Additionally, a mod ID must consist of at least two characters.

A mod ID is often a compact version of the name of the mod which makes it short but recognizable and prevents naming conflicts. Conventionally, a project named “My Project” could be called myproject, my_project, or my-project. You should usually register items and blocks (or any other things) using this mod ID as the namespace of identifiers.

Some of the starter tutorials will use a placeholder mod ID and register items and blocks under a placeholder namespace, and you can think of it as a starter template. In Fabric Wiki, we use tutorial as the mod ID. While leaving this unchanged is not dangerous for testing, remember to change it if you intend to release your project.

Tags are groups of blocks, items, fluids, entities and so on. For example, the block tag minecraft:saplings contains all of the game's saplings. In Minecraft, any type of things that has a registry can have tags.

Minecraft uses tags to determine many things. For example, blocks with tag minecraft:mineable/pickaxe can be mined quickly with pickaxes. Fabric API also provides some conventional tags. More information about tags can be found in tags tutorial and "Tag" page in Minecraft Wiki.

Fabric Loader uses fabric.mod.json to detect and load your mod. A mod usually contains at least one initializer class which should implement one of ModInitializer, ClientModInitializer and DedicatedServerModInitializer. The interfaces are all in the net.fabricmc.api package. In Fabric Wiki tutorials, we usually use ExampleMod as the mod's initializer, as done in Fabric Example Mod.

In order to change or add initializers, you need to edit fabric.mod.json and find entrypoints field, then edit them accordingly. The main block is for Mod Initializers, client block is for Client Mod Initializers and server block is for Server Mod Initializers. For more information see fabric_mod_json.

{
  [...]
  "entrypoints": {
    "main": [
      "net.fabricmc.ExampleMod"
    ],
    "client": [
      "net.fabricmc.ExampleClientMod"
    ]
  }
  [...]
}

By implementing ModInitializer, ClientModInitializer or DedicatedServerModInitializer, interfaces, you must implement an onInitialize() (or onInitializeClient() for Client, onInitializeServer() for Server) function. You can then write your codes in it.

For more information on entry points, see entrypoint.

According to Oracle's Java documentation, they are written in all lower case to avoid conflict with the names of classes or interfaces. The reverse of your domain name is used to start the names. Read more at https://docs.oracle.com/javase/tutorial/java/package/namingpkgs.html.