I can't explain SIRDS better then the writers of the SIRDS FAQ.
I'll just quote the introduction passage.
"Have you walked through a mall lately? These days, as you wander past
most of the poster shops, there will be a large group of people
staring at the same poster with surprisingly weird expressions on
their faces. Some will be in the initial stages of denial or
rejection - they will be concentrating, some slowly rocking their
heads backwards and forwards, searching for an image that they have
never seen before. Others will be grinning from ear to ear, pointing
at the poster, chuckling with their friends that a member of their
group hasn't seem them yet. "Come on Bill, come on!", they cry and as
Bill gets increasingly more frustrated he concentrates harder and
harder, until finally (if he's lucky) he sees a true 3D image."
I hope you'll be lucky :-)