What does # mean in Lua?

#is most often used to get the range of a table. For example:

local users = {"Grace", "Peter", "Alice"}
local num_users = #users

print("There is a total of ".. num_users)

Output: 3

# is the lua length operator which works on strings or on table arrays

Examples:

print(#"abcdef")  -- Prints 6
print(#{"a", "b", "c", 88})  -- Prints 4

-- Counting table elements is not suppoerted:
print(#{["a"]=1, ["b"]=9}) -- # Prints 0

That is the length operator:

The length operator is denoted by the unary operator #. The length of a string is its number of bytes (that is, the usual meaning of string length when each character is one byte).

The length of a table t is defined to be any integer index n such that t[n] is not nil and t[n+1] is nil; moreover, if t[1] is nil, n can be zero. For a regular array, with non-nil values from 1 to a given n, its length is exactly that n, the index of its last value. If the array has "holes" (that is, nil values between other non-nil values), then #t can be any of the indices that directly precedes a nil value (that is, it may consider any such nil value as the end of the array).