What is open source software? Plain-English meaning

Short definition: Open source software is software whose source code is made available to anyone to view, modify, and distribute under a license that permits those activities.

Plain-English explanation

Most software is distributed as a compiled program that you can run but not inspect or change. Open source software flips that: the underlying source code is published, so anyone can read it, learn from it, fix bugs, add features, and share their version. This openness encourages collaboration and transparency, because many people can review the code for errors or security issues. It does not mean the software is free of charge, though most open source projects are available at no cost. The key is the license, which grants permission to use, modify, and redistribute the code under certain conditions.

Why it matters

You will care about open source software when you want to trust the tools you use, customize them, or avoid being locked into a single vendor. Because the code is public, you or your team can audit it, adapt it to your needs, and continue using it even if the original developer stops maintaining it. For businesses and individuals, this can mean lower costs, more flexibility, and greater control over the software they rely on. It also matters for security: many eyes on the code can help find and fix vulnerabilities faster, though that is not guaranteed.

Concrete example

A common example is the Firefox web browser. Firefox is open source, so its source code is publicly available on a platform like Mozilla's repository. A developer who notices a bug in how the browser handles a certain type of web page can download the source, fix the bug, and submit the change for review. If accepted, the fix becomes part of the next release, benefiting all users. Similarly, a company could take the Firefox source code, modify it to include its own branding and features, and distribute its own version, as long as it follows the license terms.

Common confusion

People often confuse open source with 'free software' or with 'free of charge.' The confusion is that 'free' in open source refers to freedom to use and modify, not necessarily price. While most open source software is free to download, a company can charge for support or for a packaged version. Another mix-up is thinking that open source means no copyright; in reality, open source software is protected by copyright, and the license defines what you can do with it. For example, some open source licenses require that any derivative work also be open source, while others are more permissive.

Related terms

source code, license, proprietary software

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More context

Open-source software is often confused with freeware, but freeware only means no cost, while open-source also grants access to the source code. The term was coined in 1998 to promote a more business-friendly image than 'free software.' Many open-source projects are maintained by foundations like the Apache Software Foundation or the Linux Foundation, and they power a significant portion of the internet's infrastructure.

Additional background

Open-source software is defined by the Open Source Initiative through ten criteria, including free redistribution, access to source code, and permission to create derivative works. The model contrasts with proprietary software, where the source code is hidden and usage is restricted by licenses. Open-source projects often follow a governance structure, with maintainers who review contributions and set direction. Funding for open-source can come from donations, corporate sponsorships, or dual-licensing models. The security of open-source software is a double-edged sword: public code means vulnerabilities are visible, but also that many eyes can spot and fix them quickly. The concept of 'open source' has expanded beyond software to areas like open hardware, open data, and open science. For beginners, the best way to understand open source is to use it, read its code, and eventually participate in its community.