Learn in the textbookOpen Source Software (OSS) and Licenses
DefinitionOSS (Open Source Software) is software whose source code is made public and that anyone may use, modify and redistribute under the terms of its license.
ExplanationSource code is the text of a program written in a form people can read and edit, and a license is the set of usage terms a copyright holder grants to users. Software counts as OSS only when its license allows not just reading the source code but also changing and redistributing it. It generally comes with no guarantee of how well or how safely it works, so whoever adopts it checks the quality first.
AnalogyIt is like publishing a detailed recipe and letting anyone adapt it and share it with others, as long as they keep to the agreed rules.