In short
RTO defines the maximum time a system may take to be back in service after an outage; RPO defines the maximum amount of data loss that is acceptable.
For example, with an RTO of 2 hours and an RPO of 15 minutes, the system must be back within 2 hours and lose at most the last 15 minutes of data.
These objectives directly determine backup frequency, the need for a secondary site and the cost of the disaster recovery plan.