Free source software is software in which the source code for a given application is made freely available to the end user. I think I better define some terms. Conventional software is normally distributed in the form of executable binary. When you go to the computer store and buy Quickverse, you will only get the executable binaries. That is all you get.
Source code is the set of instructions necessary to create software applications.
Think of it this way, source code is like having the blueprints AND building material for a new house. Free source software is different from conventional commercial software. When you buy, or download, free source software, you get not only the executable binaries, but also the source code to create your own binaries. Free source is not the same as freeware or shareware. Free source is 'free' as in free speech, not free beer.