'Does the Flap of a Butterfly's Wings in Brazil set off a Tornado in Texas?' Basically it's the assumption that an apparently insignificant event may be able to bring about great change.
If you are really, really interested I suggest you get hold of a copy of Chaos by James Gleick, pub. Cardinal ISBN 0-7474-0413-5.
The first chapter deals with this topic.
(I'm not Gleick, incidently!)