A hobby is usually something that you really enjoy doing which is not paid work, although people can and do make money from their hobbies.
As opposed to your work, which you may enjoy a hobby is something of your own choosing. It is something chosen for pleaure and enjoyment. They can also provide a lot of fulfilment.
Although they are not usually done for financial gain they can lead to this for example if knitting is a hobby you could sell the finished product.
It is thought that the origin of a hobby derives from the hobby-horse. These poles were usually made of wood with a model horses head and were ridden for fun or as part of a tradition. Children still ride hobby horses which have never really gone out of fashion for children.
Having a hobby can lead to acquiring a lot of skill and, knowledge, and experience. Fulfillment is the prime aim of having a hobby.