It is from when Moses was asking the Pharaoh to let the Jews -who were at that time slaves- leave Egypt, and because Pharaoh kept refusing, God punished the Egyptians by killing all their first born sons. The Jewish families were kept safe by daubing lamb's blood on their door-posts - when God saw this blood he "passed over" those homes.

richmondparkchurch.org.uk an evangelical pentecostal church in Boscombe, Bournemouth, Dorset, UK.