What does it mean when there is a number parameter passed to toString?

The additional parameter works only for Number.prototype.toString to specify the radix (integer between 2 and 36 specifying the base to use for representing numeric values):

var number = 12345;
number.toString(2) === "11000000111001"
number.toString(3) === "121221020"
// …
number.toString(36) === "9ix"