for i, creature in pairs(creatureArray)

When looping through a table or dictionary, it returns a key/value pair. The i is considered the key, and the creature is considered the value.

Here's an example:

local t = {
    ["Item"] = "Value",
    ["Another Item"] = "Another Value",
}

for i, value in pairs(t) do
    print(i, value)
end

The output would be:

Item            Value
Another Item    Another Value