A program is free software if the program's users have the four essential freedoms:
- The freedom to run the program as you wish, for any purpose.
- The freedom to study how the program works, and change it so it does your computing as you wish. Access to the source code is a precondition for this.
- The freedom to redistribute copies so you can help your neighbor.
- The freedom to distribute copies of your modified versions to others. By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this.

But do read this, because those descriptions are short, and many people seem to interpret the 'you's as referring to the person reading the 'four freedoms' rather than the collective entity of humans, and so mistakenly believe they "don't need" free software and free software "isn't needed" because they don't know how or don't feel the need to take advantage of the freedom(s) it provides.

OT: English may be an easy language thanks to its tiny vocabulary, but trying to accurately convey anything that goes beyond the absolute basics is quite difficult.