On Ops Manager UI you noticed something like

security_configuration.trusted_certificates[N] are certificates that are configured on your Ops Manager -> Security -> Trusted Certificates
The trusted certificates accepts multiple certificates hence we can see an index .security_configuration.trusted_certificates[N] . N signifies index of the array which starts from 0.
On our example .security_configuration.trusted_certificates[3] this would be the 4th certificate from the top of the text field.