Free and open source software has literally helped accelerate growth and innovation in the global economy. Moreover, it has done so at an increasing rate as software becomes embedded in everything that we do (and is produced).
Open source software refers to software in which the underlying source code is available to see, i.e. is open. The free part refers to the licenses that accompany the software that enable anyone to freely use, adapt and build upon them. It’s free as in speech, not as in free beer (although most of this software is free in the upfront cost component as well).