What does :lower() mean?

The lower method is used to basically make ROBLOX understand something's name as all lowercase letters.

Ex:

while true do
if game.Players.LocalPlayer.Name:lower() == "bob" then
game.Players.LocalPlayer:Kick()
end
end

Btw, I've actually met someone named Bob on ROBLOX.

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lower is a method of the string class that changes all uppercase characters to lowercase.

("CAPS"):lower() is accessing the string table by the string object that gets created, so all of the member functions included in string are methods of each string object where the object itself is the first argument. ex:

print(string.lower("CAPS"))

is the same as

print(("CAPS"):lower())

and

print(("1waffle1").lower("CAPS"))

(if you can come up with a practical application for this one, tell me)

It all revolves around how literals are transformed into classes and how : is the same as indexing a function and calling it with its first argument being the object you're indexing it from.