when we run a plan, TF proposes some changes and in there it displays an option with “up-down” arrow which simply states “read”. What does it mean?
I think you are describing the Terraform Cloud equivalent of what Terraform CLI would present using the “pseudo-icon”
<-, which indeed represents the idea of “read”.
Current Terraform proposes this action only for a data resource, and only in the case where either the configuration includes a value that won’t be known until the apply step or the data resource depends on some other resource that has a pending change in the plan, and so Terraform needs to wait to see the effect of that change before reading the data source.
I get this icon for refreshing data resources during plan, not only for “will be read during apply” cases.
The Terraform Cloud web UI has some different design conventions than CLI because, as a result of being a web application, it can update in place existing UI elements to show progress over time, whereas the CLI instead streams updates only by appending to the terminal.
In particular, Terraform Cloud can report progress during planning as a grid of current status for each action being taken, and so I expect you are referring to the table row which Terraform Cloud uses initially to report that it’s in the process of reading the data source, and then that the read has completed, in situations where Terraform does have enough information to read during the planning phase. That would be equivalent to the messages reporting things like “Reading…” and then “Read complete” (or similar) in the terminal-based UI.