Gratis versus libre is the distinction between two meanings of the English adjective "free"; namely, "for zero price" (gratis) and "with little or no restriction" (libre). The ambiguity of "free" can cause issues where the distinction is important, as it often is in dealing with laws concerning the use of information, such as copyright and patents.
The terms are largely used to categorise intellectual property, particularly computer programs, according to the licenses and legal restrictions that cover them, in the free software and open source communities, as well as the broader free culture movement. For example, they are used to distinguish freeware (software gratis) from free software (software libre).
Richard Stallman summarised the difference in a slogan: "Think free as in free speech, not free beer."
Read more about Gratis Versus Libre: Gratis, Libre, "Free Beer" Vs "free Speech" Distinction, Generalizing The "Gratis/Libre" Distinction To The Open Access Movement