A Force-sensitive was a being who was highly attuned to the flow of the Force. Force-sensitives could, with training, learn to sense and manipulate the Force.
The Jedi Order took Force-sensitives at a young age (2 to 18) to start training them to become Jedi. Trained Jedi could sense high levels of the Force in persons. The presence of Force-sensitives in a vicinity could also be sensed. There were also several other testing technologies developed to hunt Force-sensitives during the Great Jedi Purge.