How can i change properties of multiple parts at once in a script?

Hello, I am currently working on making the ground parts in ma game change transparency but instead of doing part1, part2 , part3 I want to select them at once. I tried looking for the answer but it only changes to property of certain parts. I also dont understand the "i" in the loop. please help

local GroundParts = script.Parent.GroundParts -- my folder
local partsFolder = GroundParts:GetChildren() -- getting the parts



while true do
    for i, parts in pairs(partsFolder) do
            parts.Transparency = 1

            wait(15) -- 360 seconds -- 6 minutes
        end
    end