FOSS means Free and Open Source Software. Since English has two meanings for 'free' (one as in no cost, the other as in freedom) you'll also see it called libre software because Latin languages differentiate between no cost (gratis) and freedom (libre).

Open source means the source code is available and required to be accessible. The Free part is you have the right to take that code and use it in other programs. Copyleft licenses like the GPL will require any program you use that code in to ALSO be licensed under the same license, giving everyone access to your code under the same conditions. Non-copyleft FOSS licenses like the BSD licenses do not require that, and allow the incorporation of the code in closed source software.

To give an example of the differences let's use Apple. Much of OS X userspace is derived from BSD Unix, so Apple can use it and not release code. But KHTML, the KDE HTML Rendering engine they took to make WebKit, was GPL so they had to release their changes and allow others to do the same, which is how you ended up with Chrome and Edge also using and extending WebKit and now it's basically the standard browser rendering engine.