Social skills are your ability to have real life communications with others. People with excellent social skills are more likely to make friends, get jobs, and have girlfriends than those with poor social skills.
Contrary to what PUAs say, social skills cannot make up for being unattractive. Being good looking is a requirement for getting girls. An ugly or old man simply cannot get a beautiful 18 year old girl, no matter how good his social skills are.
Social skills, for the most part, cannot be improved. Your social skills are determined by how good looking you were as a kid. A good looking kid will be received well by his peers. He will grow to enjoy socializing and he will become more sociable as a result. The opposite is true for an ugly kid.