This is just a warning, it does not affect your gameplay in anyway, and you can still activate and enjoy the pack by simply clicking the 'Yes' button when prompted.
QUICK TIP: Sometimes, when activating a pack, the game may say that the pack is made for a newer or older version of Minecraft. zip file you downloaded in the first step to the newly opened 'resourcepacks' folder. – folder titled 'resourcepacks' will pop up – if there are many different versions, just choose the one you like, is compatible with your version/edition of Minecraft, the one you think your computer can handle in terms of performance.
– general recommended step for all packs, Optifine is also needed for most shaders
