"Successful and unsuccessful people do not vary greatly in their abilities" suggests that the capabilities or talents of individuals may not be the primary factor differentiating success.
"They vary in their desires to reach their potential" highlights that what sets successful and unsuccessful individuals apart is their level of motivation and commitment to realizing their full potential.
In essence, this quote conveys that success often hinges on the degree to which individuals are willing to strive for their aspirations. It encourages a focus on cultivating a strong desire to achieve one's potential, recognizing that this drive can be a defining factor in the pursuit of success.