Free software means that there are no monetary costs in order to download and use it. But there are certain restrictions in place for what you can do with free software.
Free software is a structured term developed by the Free Software Foundation in coordination with the GNU Project that might better be described as open source software. This means that the core code is publicly available for no monetary cost and that you can download it, copy it, modify it, and distribute it as long as you adhere to the licensing agreement.
There are several different licensing agreements commonly used with free software. These licenses don’t require paying any royalties or copyright fees, but they may restrict how you can distribute it. For example, WordPress is free software distributed under the GPL or General Public License which requires that any modifications or redistributions of the original code also be protected by a GPL.
The term “free” is not so much about the lack of monetary cost but rather than the end user is able to easily view the core code as well as modify it. For example, a game that is free to play would not be considered “free software” unless it was easily possible for an end user to examine the core code and make changes. If you don’t have to pay for software, but you can’t modify it, then this is called “freeware.”
Free software is considered a robust way to encourage development and rapid evolutionary progression because developers can easily access and modify the source code without being restricted by copyright protection or monetary costs. WordPress has thousands of free themes and plugins available precisely because it is, itself, free software.