This quote is making a comparison between two experiences to show how our perception of time can change relative to the situation we are in. When we are doing something enjoyable, like courting a nice girl, time passes very quickly and an hour may feel like only one second. On the other hand, when we are stuck in a bad situation, such as sitting on a red-hot cinder, then a second can feel like a much longer length of time. This quote is emphasizing the idea of relativity, which is the concept that things can appear different based on our perspective or situation.