So! You want to know more about how to create resource packs and amazing new content within them. Lovely! But we have to cover some basics first. If you don’t know anything about the structure of a resource pack and what goes where, it is recommended to check out this page on the Minecraft Wiki:
Resource pack – Minecraft Wiki
Although reading the entire article is recommended for deeper understanding of how resource pack works, the most important things to read on and understand are Behavior, Folder Structure and Template. After reading and somewhat understanding those and the basic structure of a resource pack, you can get going!
A namespace is a term that is used in computing to identify objects. Within resource packs, it is mainly used to refer to the folder that comes after assets/<namespace>. The default namespace is always minecraft, which is why in most standard resource packs, you see the folder assets/minecraft/... being used.
Namespaces can have any lower case English name. For example, if you are working on a server that has two different gamemodes, which have different assets, you could consider having the following pack structure and namespaces:
assets
| - minecraft
| - font
| - textures
| - ...
| - gamemode1
| - textures
| - ...
| - gamemode2
| - ...
When referring to a namespace in Minecraft, you have to define it before a texture path for example. When playing a sound from outside the minecraft namespace, you would use mynamespace:sound_pathsound_name where it is needed. Same thing when defining a texture path like gamemode1:customtextures/blue_ball.png.
Minecraft allows you to quickly reload your resource packs using F3+T. Using this, while editing a resource pack you will be able to quickly see your changes be reflected in the game.