This article is from the PGP FAQ, by Jeff Licquia firstname.lastname@example.org with numerous contributions by others.
This is because most people, when asked to choose a password, select
some simple common word. This can be cracked by a program that uses a
dictionary to try out passwords on a system. Since most people really
don't want to select a truly random password, where the letters and
digits are mixed in a nonsense pattern, the term pass phrase is used
to urge people to at least use several unrelated words in sequence as
the pass phrase.