Internal functions are functions written in C that have been statically linked into the PostgreSQL server. The "body" of the function definition specifies the C-language name of the function, which need not be the same as the name being declared for SQL use. (For reasons of backwards compatibility, an empty body is accepted as meaning that the C-language function name is the same as the SQL name.)
Normally, all internal functions present in the backend are declared during the initialization of the database cluster (initdb), but a user could use CREATE FUNCTION to create additional alias names for an internal function. Internal functions are declared in CREATE FUNCTION with language name internal. For instance, to create an alias for the sqrt function:
CREATE FUNCTION square_root(double precision) RETURNS double precision
AS 'dsqrt'
LANGUAGE INTERNAL
WITH (isStrict);
(Most internal functions expect to be declared "strict".)
Note: Not all "predefined" functions are "internal" in the above sense. Some predefined functions are written in SQL.
Read the upstream documentation. For corrections, first check the current version.