Each publisher gets the ability to retry sending their data if errors are encountered - something commonly seen when working with getting data from the network. The retry() operator can be used to retry the publisher a certain number of times before failing, and the catch() operator can be used to fall back to a default set of data if all else fails.
This is a companion discussion topic for the original entry at https://www.raywenderlich.com/21773708-intermediate-combine/lessons/5