proof of possession
The CSR's signature, showing the sender holds the private key for the public key in it.
Proof of possession is evidence, such as the signature on a CSR, that the requester holds the private key matching the public key it asks to have certified.
Examples
A CSR from a bench node is signed with its private key, so the CA knows the sender really holds the key matching the public key inside.
An attacker who copies a node's public key into their own CSR can't sign it correctly, so the proof of possession fails.