The actual description of what an inventor is claiming as his/her invention. Only what is in the claims are the invention. Whatever else that is discussed or shown in the other portions of a patent application, such as the written specification, abstract and drawings, if not mentioned in the claims, is not the invention and may enter the public domain. That is, if the claimed invnention was to a car with engine and wheels, but the written specification also mentioned having mirrors, the unclaimed mirrors on a car would enter the public domain and would not be protected by the invention.
However, that does not mean that someone could make a car with engine, wheels and mirrors, since that would infringe the rights under the patent for a car with engine and wheels, irrespective of what is added. It just means that anyone can add mirrors to an existing car without worrying about infringing the patent by doing so.