An externship is when you get the chance to go and learn about a job or profession by working with people who actually do that job. It's like going on a field trip but instead of just looking at things, you actually get to help out and learn by doing!
Let's say you are really interested in being a doctor when you grow up. You could go on an externship with a doctor and shadow them for a day or more to learn what they do. You might start by watching them talk to patients, and eventually you could help them by doing some tasks that they need help with, like taking someone's temperature or helping them fill out paperwork.
The important thing to remember about an externship is that you are there to learn. You might ask questions about what you are seeing and doing, and the people you are working with will teach you as much as they can. You won't get paid for the work you do during an externship, but it's a great way to learn and get some real-world experience in a job you might be interested in.