A pastime is any hobby or activity that you do to pass the time. Usually, people enjoy their pastimes.
When baseball was more popular, it was called the "national pastime," because so many people enjoyed watching or playing it. Baseball is still a major pastime, as are many other sports. Other pastimes include listening to music, playing music, watching TV, playing video games, playing with your dog, and any kind of hobby. Whenever you're awake and not working, you're probably enjoying a pastime.