Open Source software is software with code that anyone can view. A project being open source means that the creators of the project decided to make the code publicly available, often on a service like github.io (you can view this project's github page here, and also see any future projects I make on my github page).
Open Source projects are free to access, meaning that they can be a great resource for people working on other projects or learning how to code. They can also be used to ensure site transparency, for example, Mozilla FireFox is an open source browser, because people can view all of the code for the website, it would be difficult to hide any malicious intent, ware as a non-open source browser, such as Opera, are not open source, and therefore could more easily have malicious code.