The application is the IdP configuration for a specific SP but the SP is really the target application.
The documentation seems poorly worded to me, based on past experiences you should provide them with the cert I described. This is the certificate that can be used to verify the digital signature applied to the SAML assertions (by the IdP) when they are recieved by them (the SP)
Are you configuring an OAN app that has Okta documentation? or is this documentation provided by the SP that is specific to an Okta setup?
What is the app?