"Libre software" means software that respects users' freedom and community. Roughly, it means that the users have the freedom to run, copy, distribute, study, change and improve the software.
The freedom to run the program, for any purpose.
Placing restrictions on the use of Free Software, such as time ("30 days trial period", "license expires January 1st, 2004") purpose ("permission granted for research and non-commercial use", "may not be used for benchmarking") or geographic area ("must not be used in country X") makes a program non-free.
The freedom to study how the program works, and adapt it to your needs.
Placing legal or practical restrictions on the comprehension or modification of a program, such as mandatory purchase of special licenses, signing of a Non-Disclosure-Agreement (NDA) or - for programming languages that have multiple forms or representation - making the preferred human way of comprehending