A term that loosely means the ability to navigate social situations/social networks as fitting one's needs. However, the phrase, when used, covers an umbrella of different factors and skills. These include, but are not limited to, the ability to feel at ease in social situations or make those around you feel at ease, the ability to relate to those around you, the ability to converse fluidly, or the ability to grasp social "rituals" (such as what to do or say in certain situations). It is a very broad term that in and of itself has minimal literal definition, but is commonly used as an insult. Examples:
"Dude, how could you say that to her! Learn some fucking social skills!"
"I hear that guy's smart, but I bet he has really bad social skills."
It is usually taken as a given that the greater a person's intelligence, the worse their "social skills". However, this is contested, as intelligent people are often better at manipulating those around them, and are likely to have a better "theory of mind"- which means the ability to figure out what is going on in the heads of other people. (Which is a separate skill itself).
A star football player may have great "social skills" among his teammates and fans, but might become very awkward around a group of research scientists, authors, or professional dancers.
"Social skills", like "emotional intelligence", is a word often thrown around with no one stopping to figure out what it actually means.