If my ability lies in the area of writing, for example, people might call me a strong writer. If I choose to use this writing ability to write a hate-filled piece of literature downgrading some segment of the population, that would not be a use of my strength for a "right" or constructive purpose. If I use my writing ability to express ideas that allow others to explore ideas and come to consensus and understanding so that conflicts are reduced and groups learn to respect and tolerate each other, I would consider that "greatness" - not just because I was a strong writer but because I used that strength in the right way.
Here is another example of how we might apply this quote.
If Superman were blind and deaf but had all of his other super powers (flight, super strength, etc.), he would likely be unable to actually be a super hero. He would not know how to apply his strength; how to help people. His special abilities would be rather useless.
Maybe this isn't the most practical example, but hopefully it elucidates the idea behind the quote.
The idea is that greatness isn't about how strong you are but rather how you use the strength that you have. Many people are strong. Hitler was a strong speaker with a powerful regime, but he didn't use his strength in a positive way. There are war lords and war criminals all over the globe that have vast amounts of power and strength, but they are not great men. They have committed terrible deeds with their strength. A great man (or woman) uses their strength in the right way. Mother Teresa was a great woman. She used her strength to help others and to lift up instead of tearing down.
Try to use your mind... ^_*