What are .in files?

Sometimes in the sources of projects I see "*.in" files. For example, a bunch of "Makefile.in"s. What are they for and/or what does the ".in" part mean? I assume that this has something to do with autoconf or make or something like those, but I'm not sure. I've tried searching for ".in file extension", "autoconf .in file extension", "autoconf .in", "autoconf dot in", and other variants, with no luck.