https://www.ipaustralia.gov.au/ip-for-digital-business/develop/licencing-and-open-source

Is?

There are a few competing definitions for ‘open source software’. However, generally for software to qualify as open source, users must be able to freely (in relation to rights, not payment) copy, study, adapt, improve and distribute the source code of the software. 

One of the key elements to open source licences is that any changes, improvements or adaptations to the code base must themselves become subject to the same open source licence.

Open source software is mostly made available free of charge, though this does not need to be the case, depending on the definition adopted and the specific open source licence used.

Some common examples of open source licences include Australian Creative Commons licences and the GNU General Public License.