They are counting seconds to time something. It is thought it takes about one second to say 'one Mississippi'. Simply saying the number is to fast.

Yes, it's about timing. Other variations include "1thousand1, 2thousand2, 3thousand3..." (this was in a driving safety manual) or my Aussie favourite, "one-kangaroo, two-kangaroo, three-kangaroo..." :)

funny , cultural question ...I'm wondering it too...!!! :)