Free Software, or more specifically, Libre software is a term for software that chooses to give you more rights over what you can do with the software, and especially the source code of the software.[1] It does not mean that the software is free as in price, but rather it is free as in being able to modify, sell, and redistribute copies of the software.[2] Your rights are guaranteed by the license you agree to when using the software or it's source code.
The Source code is essentially the ingredients of the software you run on your computer. Most closed source software do not share the ingredients or the steps required for making the software, whether that be for financial or patent reasons. This is called Nonfree or Proprietary software. Proprietary software does not guarantee your rights, and most commonly have an End-user license agreement (EULA) which you must agree to in order to use the software.
Unlike End-user license agreements, free software licenses cannot easily be changed at random. This guards you against the terms of the sale being changed after the sale or distribution of the software.
Types of Free software licenses
Identifying Free software licenses is simple, and can be categorized into a scale, where the left most point is most restrictive, and the right most point being most permissive. Even though permissive licenses give you the most rights as a software author and developer, it does not guarantee that it won't be used against you as an end user.
For example, the GNU General Public License version 2 (GPL-2) states that all copies of the software must be under the same GPL-2 license[3], while the BSD 3 clause license allows the distribution of BSD-3 code in proprietary programs.[4]
There are also Source available licenses, which allow you to read the source, but may not allow you to redistribute or modify them. These licenses are not considered free or open, and should be considered as "most rights reserved".