TortoiseHg is a special tool that helps people who work on writing computer code manage their work. It's like a helper that keeps track of all the changes you make to your code so you can always go back to an earlier version if you need to.
Imagine you're building a sandcastle on the beach. Every time you make a change to your sandcastle, like add a new turret or moat, TortoiseHg takes a picture of your sandcastle and saves it for you. That way, if you ever decide you don't like the way your sandcastle looks now, you can go back to an earlier version when it looked different.
TortoiseHg also helps you collaborate with other people working on the same computer code as you. It makes it easier for you to share your sandcastle with others and to see what changes they've made to the sandcastle.
Overall, TortoiseHg is an important tool that makes it easier for people who write computer code to work together and keep track of all the changes they make to their work.