Open Source is a way to license software. This type of software allows you to:
- Share the software with your friends for free
- View and edit the source code
- Use the software in your own open source projects
- Normally you can download the software from a website for free
There are normally some conditions about doing these things. These conditions are usually there to protect the developers against being sued for giving you their software, or to stop other people claiming they wrote the code.