What does [i] in Lua mean?

Brackets after an object/table index on that table/object.

Something you may be more familiar with that indexes is the dot (.).

When you do game.Workspace, Lua interprets it as game["Workspace"]. That is, your asking for the value as the index "Workspace" (the string "Workspace").

Equally valid would be the following:

index = "Workspace"

ws = game[index]

-- or

i = "Lighting"
lights = game[i]

Any value except nil can be used as an index in a table. tab[i] is asking for the value at the index i. i will usually be a number, as when going through a list; for example, tab[1] asks for the first thing in the list tab, tab[2] the second, and so on.

If you had a loop, you could use tab[i] to the the ith element out of tab --- this is especially useful when i is the iterator in a loop.

Here's an example:

primes = {2, 3, 5, 7, 11, 13, 17, 19}

for i = 1, #primes do
    print("i is",i)
    print("so primes[i] is primes[",i,"] = ",primes[i])
    print(" ")
end

Which gives the following output:

i is    1
so primes[i] is primes[ 1   ] =     2

i is    2
so primes[i] is primes[ 2   ] =     3

i is    3
so primes[i] is primes[ 3   ] =     5

i is    4
so primes[i] is primes[ 4   ] =     7

i is    5
so primes[i] is primes[ 5   ] =     11

i is    6
so primes[i] is primes[ 6   ] =     13

i is    7
primes[i] is primes[    7   ] =     17

i is    8
primes[i] is primes[    8   ] =     19