The essence of this quote is that if you base a relationship on looks or appearance, eventually your relationship will fade just as beauty fades away.
In other words, if you are young and meet a very pretty person and both of you fall in love with each other but your relationship is mostly based on appearance and not of true love and commitment. Then as both of you start to grow older you will both soon realize that the love that you felt is gone and you’ve both lost the attraction that you had once felt and the relationship will crack up and there will be no more need for either of them.
For example, consider celebrities and die-hard fans. If a person who likes that celebrity falls in love with it and the famous person falls in love with that person. Then after a while the two of them break up. This happens because they were not truly in love and that they based their relationship on physical features and not on love and commitment.
To give you a metaphor so you can better understand what I am saying, consider this. You get a fancy sports car that has no dirt on it and is glamorous. You fall in love with the car and decide to buy it. Then through the years the car got dirty and didn’t really look to pleasing anymore. So you decide to throw it away.