Resource pack
Resource packs allow players to change the aspect of every texture in Minecraft without writing code also allowing for more complex cases like animations, randomness, and targeted entity changes.
Version
Resource packs are often designed to match specific game version. Updates may introduce new or change current blocks, textures, or mechanics that will may not be compatible with older packs but it will only result into rendering the default textures instead without causing any problems so it’s safe to treat resource packs versions as an indicator to “works the best with” and not an actual requirement for the pack to work differently from Mods where the version is an actual requirement and it will not even load Minecraft at all.
Server resource packs
It’s possibile to load resource packs on the server so every player has them when they join the server without installing them on their Client.
Resources