Midi-chlorians are microscopic life-forms that live in the cells of all living things, which create an energy field known as the Force. A very small percentage of those living things are able to hear them "speaking" and thus tap into the Force and do cool stuff with it. The more midi-chlorians one has, the more likely one is to hear them and the more powerful one will be. For some yet undiscovered reason, these Force-sensitive individuals almost exclusively exist within the Lukassi galaxy, though individuals transplanted to the Milky Way galaxy during the Great Lukassi-Milky Way Transgalactic Migration, along with their descendants, retain their abilities.
Now of course you're probably wondering, why not just inject additional midi-chlorians into your own bloodstream to become more Force-sensitive? You think you're so smart, don't you? Well, lots of people already thought of that, and they tried it, and it was a terrible idea. Many of the recipients became mentally unstable and some of them turned to evil. In the rare cases where it did work, there were negative side effects like excessive twitching. Similar things happened to clones of Force-sensitives. You just can't play God with things like that.
Prior to the discovery of midi-chlorians, a lot of people thought the Force was just magic or something. After their discovery, a lot of people got really upset and thought they were dumb, which would hurt their feelings if they had any.