A jewel is a cut and polished gemstone, either precious or semi-precious, that can be used to make jewelry. Another word for a jewel is a gem.
A jib is a smaller, triangular sail at the front of a sailboat. Some larger sailing ships have more than one jib.
A jigsaw puzzle is a picture that has been cut up into lots of little pieces. Then you have to put all the pieces back together to make the picture. The jigsaw pieces are usually cut into special shapes like in the picture, so they fit together tight and stay together as you are working. Jigsaw puzzles usually have between 100 and 2000 pieces, and can take a very long time to put back together.
A job is a task that people do, usually for money. Some jobs are small, like mowing the lawn, and some jobs you might do every weekday for many years, like working in a factory.
A jockey is someone who professionally rides horses. Jockeys usually ride horses in races. Jockeys need to be as light as possible, so the horses can run faster when they are carrying them.