simple but kinda noobie question?

It's part of a for function. Ex,

for i = 1,5 do
    print(i)
end

i will go from 1-5 in increments of 1, its default value.

Also i is just a variable, you can name it anything.

An example of a use for it,

for Variable = 0,1,.1 do
    ScreenGui.Frame.BackgroundTransparency = Variable --Changes the transparency to the variable that goes from 0 to 1 in increments of .1
    wait()
end

Also you can use it with tables,

Table = {1,2,3,"Bleh"}
for i = 1,#Table do
    print(Table[i])
end

CFrame example as requested,

for i = 0,15 do --Run 15 times, 0-15 in increments of 1
    Part.CFrame = Part.CFrame + Vector3.new(0,.2,0) --Add .2 studs to the CFrame of the part, every time this for loop runs, the part will go 3 studs all together (.2 * 15 = 3)
end

More information, RobloxDev ForLoops