A hobby is something you like to do (or something you like to play with) for fun, or for relaxation, when you have free time and you're not working or studying. Hobbies bring you joy and fill your time.
Your hobbies might include painting, reading books, playing a musical instrument, building things with Legos, or carving figures out of wood. Or, your hobby might be collecting something. You might collect old stamps or interesting rocks; if you do, then your hobby is a lot of fun for you as you find, organize, and play with your beloved items.
(Source) My grandfather's hobbies included making wine and building model airplanes. My hobbies include reading books, playing the ukulele and the piccolo guitar, and learning new ways to style hair and makeup. You can always ask someone about their hobbies: it's a good way to get them talking, and a good way to get to know them and the things they love to do.