There are certain commonly used open-source licences that are often applied to software that has been made available in source code form to the public for free. Those licences fall into two general categories: "permissive" licences and "copyleft" licences.
Popular "permissive" licences include:
These commonly used permissive open-source licences typically impose few restrictions and requirements on licensees and permit them to use the permissively licensed open-source software in products made available in executable form or over the internet that are subject to proprietary (i.e., non-open source) licence agreements. Many open-source licences, even permissive ones, include attribution requirements.
Popular "copyleft" licences include:
In contrast to permissive licences, copyleft licences generally allow licensees to use, reproduce, modify, and distribute open-source software, but there's a catch: all distributions of the original open-source software, and all modifications to that software, must be provided in source code form and under the terms of the copyleft open-source licence. In the open-source context, "distribution" generally refers to transferring a copy of the open-source software outside of your company (e.g., as embedded in hardware, or in software applications that are intended to run locally on a user's machine).
Under some commonly used copyleft licences (e.g., GPL version 2), when copyleft open-source software is distributed and that copyleft code integrates or interoperates with your proprietary code in certain ways, that proprietary code is deemed subject to the terms of the copyleft open-source licence and must be made available in source code form. Some of these particularly onerous copyleft licences even treat copyleft open-source software used over a network (e.g., on a software-as-a-service basis) as a distribution that could require proprietary code to be subject to the terms of the copyleft open-source licence and made available in source code form.