A patent is an exclusive right to exclude others from making, using, or selling an invention, granted by a government office tasked with this chore.
Albert Einstein, famously, was a patent clerk, which just shows to go ya.
Patents vary in the time period for which they are honored and what efforts the owners must go to in order to retain them. Usually they are only granted for new, useful, and non-obvious inventions. Obviousness is determined from the standpoint of a person having ordinary skill in the art.
Patent litigation is hard. Patent lawyers often know science; this is in fact a requirement in some countries.
 Patents and conspiracy theories
Patents figure in many conspiracy theories, especially those regarding automotive woo. A typical argument goes that we cannot buy cars with, e.g., 200-mpg carburetors because oil companies have bought the patents. That argument suffers from many flaws:
- If true, it would be insanely easy to prove: Just give the patent numbers.
- Such rumors have been around long enough that the patents would have expired by now.
- Since few if any inventors deal with the bother and expense of obtaining patent protection around the world, people at least somewhere in the world should be free to buy such cars.