The open closed principle is the O in the SOLID principles. It was created by Bertrand Meyer and states:

Software entities should be open for extension, but closed for modification

Let's dive deeper with an example.

We've been tasked with creating a platform that merges all video streaming sites. The platform will be able to add a platform to its list of platforms, search for a video on all platforms and play a video from the search results. The class can look like so:

CentralVideoStreamingPlatform covers most mainstream video streaming platforms. However, if we were to add another platform, say DisneyPlus, another #add() would be created. This directly violates the open for extension aspect of OCP.CentralVideoStreamingPlatform must be open to accept different details without needing to change code that's already written. By overloading #add() again, we are needlessly changing CentralVideoStreamingPlatform. Who's to say how many #add() CentralVideoStreamingPlatform would have as more streaming platforms are added?

To ensure CentralVideoStreamingPlatform's extensibility, we can refactor the video streaming classes to implement a VideoStreamingPlatform interface and use one #add().

In Conclusion

The open closed principle is a great principle to keep in mind when writing classes.