A jig is like a special helper that helps you do the same thing over and over again really easily. It's like having a friend who always knows where to put things so you don't have to think about it.
For example, if you wanted to make a lot of small wooden boxes that are all exactly the same size, you could use a jig. The jig is a tool that you attach to your saw or drill, and it helps you cut or drill in exactly the right place every time. Instead of trying to measure and mark each box individually, you can use the jig to do it for you.
Another example is if you wanted to make a bunch of holes in a piece of metal in a specific pattern. The jig would hold the metal in place and guide your drill bit exactly where it needs to go for each hole.
So, a jig is like a special friend that helps you make things faster and more accurately by holding things in place and guiding your tools.