This is a famous quote attributed to Albert Einstein, one of the most brilliant minds of all times. He was passionate about wanting to see a better world. He was practical and took advantage of his fame to encourage people to live better lives.
It is from that feeling of sharing practical advice to others that this quote stems from. Albert Einstein was one of those people who was years ahead of his time. He predicted how cell phones would turn us into mindless zombies. He lived through World War II and was an important figure who addressed President Roosevelt about the dangers of nuclear technology.
When stating that a person should strive to be of value, this phrase tries to establish that this is a more excellent achievement than success. Being of value means that you are doing precisely what Einstein is well known for: encouraging others to live better. Being of value means that you are not enveloped by a selfish sentiment of stepping on others in order to reach your goals.
To some, being successful represents the more shallow desire of being able to fulfill your own dreams even if this means undermining others around you. A successful person is one that has fulfilled their own desire, but has not necessarily contributed to bettering their surroundings. When success is your ultimate goal, it usually becomes a selfish and relentless enterprise. The measurement of success is not a measurement of how valuable a person is.
One must strive to contribute to those around us in a positive way: to encourage, to motivate, to inspire, to make the world a better place. When you strive for these things, then you can say that you have successfully taken a path of selflessness and have achieved true value.