Pre-warp civilization is a term used by the Federation to define any society that has not yet discovered and utilized faster-than-light space travel like the warp drive. Pre-warp cultures can range in technology level from extremely primitive to that of Earth in the mid-21st century. (Star Trek: First Contact)
The Prime Directive of the Federation interdicts any disturbance of such civilizations, including any form of contact, until a culture has independently proven its warp-capability. Otherwise, any involvement in the development of pre-warp societies could result in the contamination of their culture, thereby altering their natural evolution. Usually pre-warp civilizations are limited to their homeworld. However, their technical abilities can be sophisticated enough to expand throughout their solar system. (ENT: "Dear Doctor")
The Federation does allow limited covert research to be conducted on pre-warp civilizations, usually by means of a duck blind. (TNG: "Who Watches The Watchers") Observers may also be placed amongst a civilization as a prelude to first contact. (TNG: "First Contact")