According to Webopedia.com, "open source refers to a program in which the source code is available to the general public for use and/or modification." Individuals and companies who choose to follow the principles of open source are agreeing to share the computer language (source code) they have used to create a computer program or software.
The practice and utilization of open source is helpful for the programmer to receive quick feedback on his source code, creating a fast problem-solving system where others can help detect any problems, or "bugs", in the program. Sharing source code is also helpful to others because viewers are free to borrow the code to expand on the idea or to create a new program inspired by the original code.
Here is analogy to help you grasp the idea of open source: imagine you were developing a medicine and decided to share the ingredients and your research with the world. Now billions of people all over the world have access to the information. These people can now give you feedback to help hasten the success of the medicine, which will help cure sick people, or they can take your ideas and develop their own medicine, which will still help sick people. Open source is similar to this scenario, they are both giant brainstorming sessions.
Two open source softwares you may be using everyday include the web browser Mozilla Firefox and Google's Android platform. By following the principles of open source, these two softwares are encouraging people to create add-ons and applications for their products. This helps create a bug-free, fuller experience for everyone.