What does a photo caption mean in GM?

Question

Node.js

Tell me what the photo signature
is. Using nmp gm, I get the photo data using identify, it has a Properties signature field of the form

88b510b5488f1fbf096b6c867133b84005b9e1b50f3cef3d93921be66653a477

1. What is the hash of the photo? What does it come from?
2. I need to do a simple check for duplicate photos, can I use it in this case?