From the man page:

  The mimencode	program	simply converts	a byte stream into (or out of) one of
  the standard mail encoding formats defined by	MIME, the proposed standard
  for internet multimedia mail formats.	 Such an encoding is necessary
  because binary data cannot be	sent through the mail.	The encodings under-
  stood	by mimencode are preferable to the use of the uuencode/uudecode	pro-
  grams, for use in mail, in several respects that were	important to the
  authors of MIME.

Mmencode is part of metamail, and can be installed as part of that package.
It is provided here as an independent package since some programs require
mmencode only and hence you can avoid having to install the entire metalmail
package when not required.