What does this error message mean?

FYI. If you are just looking to see if any process errors are reported during the backup process, try this command in the Terminal:

• sudo log stream --predicate 'subsystem=="com.apple.TimeMachine"' --info <return>

A "good" result will be when the last line item in the results reports: Backup completed successfully

Not an error message. The message is telling you that your Mac and Time Capsule are sending data and replying to each other.

No need to watch the progress unless you have the extra time to sit for a number of hours. On average, Time Machine will back up about 30-35 GB of data per hour. But, the process starts slowly for first 20-25% of the backup, then picks up speed, then slows down again toward the end of the backup.

Here, we assume that your Mac is connected to the Time Capsule using an Ethernet connection. If you are using WiFi to back up, the process usually takes longer.

Once the backup is done, all the data on the Time Capsule must be indexed, so you may not be able to run another backup for a few hours or more after the initial backup is done.

Unfortunately, Time Machine is prone to corruption issues since the introduction of the Sierra operating system, so when.....not if.....you see the message below, you will know that your backups have become corrupted. Then, you need to erase the Time Capsule and start over again.