(You should read the description of "nothing" after you have already read the descriptions of the other hand rankings.)
If you don't have any higher ranking hand, then you have "nothing". The cards below represent a hand that would rank as "nothing":
It isn't a "Pair", it isn't a "Rainbow", it isn't "Two Pair", it isn't "Three of a Kind", it isn't an "Straight", it isn't a "Flush", it isn't a "Full House", it isn't "Four of a Kind", it isn't a "Straight Rainbow", it isn't a "Straight Flush", it isn't "Five of a Kind", so what is it? Nothing!