Definition: “Short-term experience of employment, typically arranged for older pupils by schools.”
To me, Work Experience is extremely valuable. It is initially something that helps you to get a job when you need one. You want to be a journalist? Then having work experience with a magazine for a couple months where you helped write for their magazine would give you a good chance of getting that job. You want to be a teacher? Then having work experience in a primary or secondary school teaching a year group would help you on your way.
It can be quite challenging though, as it often involves no pay and quite a lot of work as you are there to gain experience. However if you are really committed to learn more about your chosen field than this is the best thing you could do.