_ is a typical name to use when the language requires a variable name that the program doesn't otherwise need. In this case, the program just needs to run some code four times, and doesn't care about the index. The language doesn't have a plain "run this code X times" construct, but it does have a "iterate over this list" construct, so the code iterates over the list [0, 1, 2, 3] and puts the current element into a dummy variable since it's not being used.