There isn't one social skill; there are many. They involve actual skills, knowledge and beliefs about self and the world that come together to make people better able to manage relationships:
Socially skilled people tend to be confident people. They feel generally good about themselves. While they know they aren't perfect, they do feel they are "good enough", and worthy of other's love and caring. They expect that other people will probably like them if they give them a chance.
Socially skilled people tend to be outgoing and positive about life. They take the initiative when meeting other people, and make it clear, through their body language manners and words that they are in a good mood, or (and this is the important part) at least willing to look for the good in a situation when they are not feeling so hot. Other people tend to find these qualities to be rewarding and want to be with such people. Less socially skilled people tend to be less outgoing, and frequently less positive when they meet others. They may be perfectly nice, but no one gets to find that out, because they do not tend to introduce themselves, but instead wait for others to introduce themselves.
Socially skilled people tend to have a good grasp of social protocol. A protocol is a shared convention for communication. Protocols allow two people (or groups) from different backgrounds who may have very little in common to know how to speak with one another without anyone getting offended. For example, socially skilled people understand what other people are expecting to hear when greeted or approached, or how they expect to be thanked when they have given a gift. Because they understand what to do, they are able to produce desired responses that those people want and expect. They know, for instance, to shake hands when meeting a business partner, kiss on both cheeks when meeting a European friend, and make a special hand sign or give a hug when greeting a close childhood friend. Other ways that people express their command of social protocol might include that:
They smile and share their good mood with others.
They know how to "small talk" and don't find it offensive or demeaning to do so. They understand that it is inappropriate (and often frightening) for people to share too much too soon. Small talk is a way of sharing very little, but still expressing interest in another person. After a little small talk, people feel more comfortable, and (depending on the relationship and the situation) deeper subjects may be brought up.
They ask the people they interact with about themselves, expressing interest in their life and interests. They know that people like to talk about themselves, and will typically appreciate the audience.
They use body language to communicate their interest:
They lean forward slightly rather than reclining backwards
They look at people when they talk to them, making eye contact frequently
Their arms and legs are open, rather than crossed and closed.
They do their best to remember the contents of conversations, and show people they remember when they meet again. They know that people are appreciative of being remembered.
They are polite. For example, they say, "Thank you" when someone makes them a compliment, and "I'm sorry", when they want to express concern or apologize.
They make sure they are reasonably well groomed, so that people don't look at them and form a negative first impression.
They behave reasonably well, showing awareness that they are in a public place. For example, they don't pick their nose or scratch their buttocks.
They are willing to be vulnerable as becomes appropriate to the situations they find themselves in. They aren't closed people, but instead are willing to share themselves appropriately. They are sensitive to the possibility of oversharing (saying too much, too soon), and avoid doing that.
They are careful when choosing partners for long term relationships. They do not commit themselves quickly or easily, but rather take time to get to know the character of the people they are considering. They understand the importance of partner compatibility in keeping long term relationships healthy. This is to say, they understand that partners' values, fighting style and ways of expressing themselves need to mesh well if relationships are to work well. They understand that compatibility is separate from love, and that while two people may love one another, they may not be good long term partners for one another if they are incompatible.
Social skills are, in essence, the personal skills equivalent of marketing skills in business. You may be a wonderful person, but if you don't know how to present yourself to people in a way that helps them want to "buy into you", you won't find yourself with many "customers" (e.g., friends). There is a competitive aspect to many relationships, especially when those relationships are new. A good command of social skills helps you to put your best foot forward, and be maximally proactive. When you have good social skills, you have the tools necessary to make relationships happen.
