Assimilation was the process by which the Borg added new members and new technology to the Collective.
Borg assimilation was predatory; every species the Borg encountered was assessed to determine whether its biological and/or technological distinctiveness was considered "worthy of being added to the Collective's perfection". If found to be so, the species was set upon and forcibly assimilated; the Collective considered the species' will in the matter "irrelevant". As of the 24th century, the only species known to have been considered unworthy of assimilation were the Kazon.
The Borg generally did not assimilate individuals, and instead preferred to target larger groups such as the crews of starships and the populations of planets. When in the presence of a small number of individuals, Borg drones would ignore them altogether. This policy would remain in effect until an individual demonstrated some quality found worthy by the Collective, or posed a threat to Borg activities. For example, when the USS Raven was first detected by a Borg vessel it was disregarded. By the time the Raven was detected a second time, its crew had developed technologies to elude Borg sensors, making them a more worthy target.