Personality development means to develop & improve upon one's capabilities and skills. These capabilities are not the usual maths or science knowledge related. Personality means how one behaves, organizes, appears, talks, dresses, thinks, acts etc.
Personality means confidence, spoken language skill (to be able to speak clearly, confidently, with out mistakes etc..), thinking logically, ability to convince others by reasoning, ability to impress others, charm in the appearance, good dressing style, ability to perform in tough situations, being honest, being kind to others, observing manners, being cool in difficult situation.
What you are, other than you subject knowledge contributes to personality.
Develping one self to be a superior person and effective person is personality development.