Open-source software(OSS) is computer software that is released under a license in which the copyright holder grants users the rights to use, study, change, and distribute the software and its source code to anyone and for any purpose. Let me put it in simpler words, open source is sharing your code with the world and receiving help/modifications from developers all around the globe. Now you might be thinking, it would become messy. Well, no. If you are the repository manager, you get a chance to choose which changes you want to see in your project and would help you out. And you can also revert back the changes if you don't like them. Sounds interesting, doesn't it?
Some open source softwares are Microsoft's Visual Studio Code, code editor, Mozilla Firefox, the web browser, MySQL, open source database management software and many others.
Open-source software may be developed in a collaborative public manner. Any one can collaborate and make pull requests but make sure they are genuine. You can use GitHub, which offers the distributed version control and source code management functionality of Git, to be a part of the open source community. Other options are GitLab, Bitbucket, etc.
Keep Learning! Keep Coding!!