The "=ARC= NATO Offensive" (ARC:NO) mod aims to add content to the blufor side of Arma 3. More exactly, the mod add new units and vehicles from differents NATO countries.
From a technical view, this units and vehicles use the 3D models of the vanilla game with new HD textures.
You have to keep in mind that this is only re-texturation, so some units will use clothes and gear which are not used IRL.
The mod is splitted in "packs". (So people can take only units they wants etc.)
To install =ARC= UK INF Textures Pack you should use modfolders to keep it seperate from the official game content to prevent issues.
With Arma 3 you can use different ways to set up your modfolders to use custom content you have downloaded.
Please visit the Arma 3 Mod install instructions page
for more information about using custom mods and addons in Arma 3.