What is "[ ]" operation/function used for in lua (easy question)?

"Complex" or "compound" values in Lua are made with tables.

Tables are a group of key => value pairs. For example, you could have a table mapping fruits to their prices per pound:

local price_per_pound = {
    ["apple"] = 4.50,
    ["banana"] = 0.58,
    ["strawberry"] = 2.32,
}

A table associates a value with each key.

In the above table, "apple", "banana", and "strawberry" are the keys. The value associated with "apple" is 4.5. The value associated with "banana" is 0.58, and the value associated with "strawberry" is 2.32.

You can read the value associated with a key from a table using []:

local fruit = "banana"
print(price_per_pound[fruit]) --> 0.58

You can also associate a new value with a key using []:

-- Bananas got pricier!
price_per_pound["banana"] = 0.63

Lua provides a convenience syntax for []. If the key is a string that makes a valid variable name (e.g., "granny_smith" but not "granny smith", which has a space), instead of writing ["granny_smith"] you can write .granny_smith:

price_per_pound.granny_smith = 1.49
print(price_per_pound.granny_smith) --> 1.49
print(price_per_pound["granny_smith"]) -- 1.49

You can also drop the [" "] when defining a table with these keys:

 local price_per_pound = {
     banana = 0.63,
     granny_smith = 1.49,
 }

For example, if you write workspace.BlueTaslem to refer to my player's character, you could instead write workspace["BlueTaslem"].

When you're trying to refer to objects that have spaces in their names, you have to do this, since spaces mean the name isn't a valid variable name:

workspace.BlueTaslem["Left Arm"]