So you know how our bodies have this red liquid called blood that helps us stay strong and healthy? Well, when people are jumping out of flying planes, they sometimes sing a song about "blood on the risers."
But don't worry, no one is actually bleeding! The "risers" are the straps that attach the parachute to the person's body. When they land back on the ground, sometimes the straps can rub against their skin and make it red and irritated, which is what the song is talking about.
It's just a silly song that people like to sing to pass the time and have fun while they're jumping out of planes!