The package redefines \makeindex, \index and \printindex commands to deal with multiple indexes. In each case, the command gains a (first) argument specifying the index in question: the name used there is used as a name for the index files, so one then uses the command
makeindex <indexfile>
for each index, separately.

The package does not work in documents using AMS classes; the AMS provide a replacement package amsmidx, which is based on multind.

See also index splitindex.

The author is F.W. Long.