Opensource means that they hold an opensource license. This means that unlike such things as windows, which is proprietary, the holder of the license must, amongst other things, make the source code known. You are allowed to make changes to the code, and your own add ons or developments but by doing so you are bound to return the changes, so that the software does not splinter like Unix did. That way if your changes are good, they will be included in the next release.
Regular licenses do not allow such changes.
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