The skill to behave among and interact with other people without getting on their nerves. A person with high socialisation skills is perceived as a charismatic and interesting person. This skill allows you to navigate social situations, get on people's good sides and avoid pissing people off.
In addition, this covers such actions as convincing others to take your side, charming someone, bartering for prices and showing the proper etiquette depending on social context. A person growing up in the slums is probably more familiar with streetwise, while someone born with a silver spoon in their mouth is more familiar with social etiquette of the affluent.