[lukevp]

Not all values are known to you or terraform until after the apply. For example, on AWS, publishing a new version of lambda might have the ARN as known after apply since it includes the version # as part of the string. Or something like the calculated hash of an S3 object that would be known after apply. You can still reference these values in other parts of your terraform, and it will propagate the < ‘known after apply’ state there too. These dependencies are built into a graph to determine which items should be applied first, so all references to unknown data can be resolved in the correct order.