Force-sensitive is the term used to denote people who had been gifted with the ability to use the Force. While all living beings are connected to the Force, the term "Force-sensitive" is only applied to the those who has the potential to use the Force in real and tangible ways.
The degree of Force-sensitivity in a being could be gauged by their midi-chlorian levels, though this is not the only way to detect such beings. Those strong in the Force and trained it it's use could sometimes sense Force-sensitivity in others by simply being close to them.
Among those who were Force-sensitive, the range of sensitivity varied greatly. Those with a high degree of sensitivity has the potential to become beings of astonishing power. Those on the lower end of sensitivity has a low probability of ever being able to use any Force powers properly. But they still might find themselves to be unusually "lucky" or highly talented in some area to a degree beyond mere skill.