Yes, its open source. The tools we use (Perl, PostsgreSQL, Apache ++) are also open source.
Open source means that the source code of the software is available for free to everyone and you can modify the code and distribute it yourself. https://en.wikipedia.org/wiki/Open_source
Look for a place to contribute. This means not only programming, but also documentation, theme design, art work, or architectural design.
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GNU GENERAL PUBLIC LICENSE Version 2
https://github.com/ledgersmb/LedgerSMB/blob/master/LICENSE