Open source software is any computer program whose source code is made available for anyone to inspect and modify. Open source is basically a philosophy of being able to share and modify something because its design is made accessible to everyone.
Normally, only the original software developers get to see and make changes to a program’s source code. Open source software removes this restriction so that anyone can improve that program by adding features to it or fixing parts that don’t always work correctly.
Say, the special stew they serve at a nearby restaurant is really good, although it’s just a little bit too salty for you. If you had access to the recipe you could prepare your own version of the stew with a little less salt in your own kitchen. That’s the essence of open source.
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