Free and open-source software (FOSS) is software that is developed collaboratively by programmers without restrictions on modifications or improvements. FOSS has grown because users can access source code and help fix bugs and improve programs. Examples of FOSS include Linux and GIMP, a free photo editing program. The document discusses FOSS terms, why FOSS has grown, pros like being free and allowing continuous improvements, and cons like some users finding it confusing without paid support.