It is generally a term used to refer to people who are incapable of empathy. Someone without empathy can’t share the emotions of others, or imagine another persons emotional perspective.
By this definition if Bob is a sociopath, then seeing Laura in pain, or sad, or grieving will either make Bob feel good, or it won’t make Bob feel anything at all.
Sociopaths care about people the same way people care about things. A sociopath decides to hurt you, comfort you, ignore you, kill you, or save your life in order to serve his own interests.
Some make this strategy work better than others, and a large number find themselves in jail or dead. More competent ones lead very successful lives, and typically perform well in certain roles.
You might well want your surgeon or pilot to be a high-functioning sociopath. Your lover? Maybe not so much.
When most people think of sociopaths they think of serial killers, but this isn’t always, or really even often the case.
Many serial killers can be described as sociopaths, but the majority of people who could be considered sociopaths are not serial killers.
There is no real consensus in psychology, but it is often associated with various social/personality disorders, especially anti-social personality disorder, and is sometimes used synonymously with psychopathy.