The readahead reads the contents of a list of files into memory, which causes them to be read from cache when they are actually needed. Its goal is to speed up the boot process.
Not to be confused with http://code.google.com/p/sreadahead/ which got added today as sys-apps/sreadahead. ;-)
"readahead was obsoleted by systemd-readahead, which is part of systemd."