This article is from the Macintosh system software FAQ, by Elliotte Harold firstname.lastname@example.org with numerous contributions by others.
Easy Access is a *WONDERFUL* system extension from Apple, useful
for far more than its intended purpose. Unfortunately it's also the
source of a lot of confusion and strange behavior on many Macs. It's
even been suggested that anti-virals should detect and report the
presence of Easy Access since it produces more false virus reports
than any other software in Macintosh history. If you're using
System 7, your Mac will emit an ascending whistle for about two
seconds when Easy Access is turned on and a descending whistle
when Easy Access is turned off. You may also hear a beep after
Easy Access has two pieces, Sticky Keys, which is turned on by
hitting the Shift key five times in a row without moving the mouse,
and Mouse Keys which is turned on by hitting Command-Shift-Clear.
Sticky Keys lets you type things like Command-Shift-Clear without
doing the Rose Mary Wood shuffle. Just hit the modifer keys you
want to use and then hit the regular key. For example if Sticky Keys
is turned on, you could also turn on Mouse Keys by typing Command,
then Shift, then Clear rather than by hitting them all at once.
When Sticky Keys is turned on an icon appears in the menu bar
to the right of the application icon/menu. Mouse Keys lets the
numeric keypad substitute for the mouse. This is especially useful
for making precision, single-pixel adjustments in draw and paint
programs and for safely shutting down or restarting your computer
when the mouse is frozen.