Go to the Advance settings tab of the Drupal SP module.
Custom Certificates are used as an encryption certificate to be provided by the SP side to your Identity Providers.
Although by default we already have a self-signed certificate generated, you can use your own customized certificate to increase the security of configuration.
You just simply need to copy and paste your own Public key and Private key here and upload it.
You can also generate your own key pair of certificates by clicking on the Generate Self-Signed Certs button.