To confirm that Jesus was dead, a Roman soldier stabbed his side with a spear.
blood and water flowed out: This has several levels of meaning:
The spear probably cut through the pericardium, the sack around the heart, releasing these fluids.
John might have been thinking of more Passover symbolism. The Passover lamb’s blood had to flow as it died.
The water flowing from the side of Jesus is similar to the language of "living water" that Jesus had mentioned earlier (see 4:10–14; 7:37–39).