This resource pack doesn't do anything on it's own, but when loaded with the Better End Sky mod with either the forge version or the fabric version, it changes the sky to look like this:
The resource pack is for versions 1.1 and 1.2 of the mod.
If you want to use the resource pack with version 1.0 of the Better End Sky mod, you need to unzip it and rename the file at assets/minecraft/textures/environment/ named custom_end_sky.png to just end_sky.png.