A hobby is basically an activity you do in your leisure time. They can be anything from painting, playing basketball or even gardening. It is something that relaxes you, helps you be calm and have a particular amount of time dedicated to yourself.
Picture 11:03 pm on the clock, everyone asleep or at least trying to sleep. There are a couple of books stacked on the right side of your table, a laptop open with music playing in a single window and your subject notes assembled on the other.