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 in some cases, my-project also works, but dashes in modids can be a slight pain to deal with at times [citation needed]. This mod would register items and blocks using this mod ID as a registry namespace.

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, or fluids with similar properties, i.e. minecraft:saplings contains all of the game's saplings. Information about what to call tags for your mod can be found here.

Read more on what tags are on the Minecraft Wiki

Fabric Loader use fabric.mod.json to detect and load your mod.

A mod usually contains at least one initializer class which should implement one of ModInitializer, ClientInitializer and ServerInitializer. The interfaces are all in the net.fabricmc.api package. In order to change or add initializers, you need to edit fabric.mod.json and find entrypoints block, then edit them accordingly. main block is for Mod Initializers, client block is for Client Mod Initializers and server block is for Server Mod Initializers.

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

By implementing Mod Initializer interfaces, you must implement an onInitializing() (or onInitializeClient() for Client, onInitializeServer() for Server) function. You can then write your codes there.

Also, there is a block called initializers.

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.