what is "[i]" and "[[i]]" Used for?

In a table, you can take the table's name followed by brackets and a number, and that gives you the value corresponding to the number, like this:

MyTable = {"Hi","People","I'm","Perci1!"}
print(MyTable[1])
print(MyTable[3])

--output: Hi, I'm

If you have a variable for a number and that variable is changing, like in a for loop, then you can affect every value in a table in a very short amount of time.

MyTable = {"Hi","People","I'm","Perci1!"}

for i = 1,#MyTable do --#MyTable is the amount of values in the table. In this case, 4.
    print(MyTable[i]) --Since i changes each time, we will print all the values in the table one at a time.
end

--output: Hi, People, I'm, Perci1!