What does it mean to say "a patentee can be his own lexicographer"?

This particular expression means that the patent applicant can devise or create his own vocabulary to describe the invention.  New words can be created, or words can be used in ways that differ from their ordinary sense.  However, if the patent applicant uses words in a specialized or unusual sense, that sense must be made clear and unambiguous within the specification of the patent.  Further, words in a patent claim cannot have a secret meaning (Intellicall, Inc. v. Phonometrics, Inc., 952 F.2d 1384, 1387-1388 (Fed. Cir. 1992)).