What is the /Accoutrement Class - Please explain in Details; I've looked at the wiki?

Accoutrement Is clothing for your head. Your face and head do not count as Accoutrements though because face is a Texture and a Head is a basepart.

The section below isn't about accoutrements but it uses the accoutrements class.

You can use the method :IsA() to find out if something IS A certain class. If it's right then It will return true. If not, it could return false. You can use this in scripts to delete hats when they join the game.

game.Players.PlayerAdded:connect(function(plyr)
    plyr.CharacterAdded:connect(function(char)
        local body = char:GetChildren() --The characters body parts.
        for i = 1,#body do
            if body[i]:IsA("Accoutrement") then --If there is clothing on the players head
                body[i]:Destroy() --Delete the accoutrements.
            end
        end
    end)
end)

I agree with Percil though, the wiki is pretty clear.