Hello, Dariana. A harness is a set of straps and fittings used to secure something, in this case, it's the angel wings. The plastic harnesses are securing the angel wings to everyone's backs. I hope this explanation helps. If not, please refer to the pictures which you can find at the following link: https://www.healthypets.com/angel-wings-harness-costume-small.html. It's a silly picture of a dog's angel wings costume, but it shows the detachable harness, which is somewhat similar to the harnesses the people in the passage would wear.

If you google "harness for wings" you will be able to see what is being discribed. I hope this helps.

A harness is something strapped around a person's body that will allow them to be fixed to something else. For example, if a person could have a harness to attach to a rope if they were climbing a building or rock climbing. A harness would be used to attach a person to a parachute. Sometimes the words "baby harness" are used for a harness that straps a baby into a car seat, or allows a young todler to be on a leash. In this example, it appears the plastic harnesses allow people to wear the wings of angels as part of the costume for fans of the movie. The harness allows the wings to be fixed to th body.