"No messenger will aid you until you have discovered their resting place"

Sometimes at the beginning of a puzzle, there is a white-board I can click on. When I click on it, my character leaves a message asking for help. The reply is always the same:

No messenger will aid you until you have discovered their resting place. Return here then to seek their counsel.

How do I "discover a messenger's resting place?" And what exactly do these boards do?

Solution 1:

Though @janb already answered it. I would like to add more details.

1. White-boards are used for small hints, like "Try to make it as high as possible". Only 3 hints are possible in the game, as far as I know. (You can also redeem three extra hints by playing the (free) promotional game "Sigils of Elohim" and redeeming the codes it gives you for those hints. (с) Jonathan Thiele)

2. They are present only at puzzles with red sigils, as far as I noted. But all those puzzles are totally managable with out hints.

3. There are 3 messenger's resting places, one per hall. They are behind boarded up doors.

4. You can unlock the doors only when you reach hall-C. To reach it you need to collect all green sigils at halls A and B.

Solution 2:

The white-boards are the implementation of a hint system. If you click them, you can see hints for the puzzle. However, this hint system only unlocks in World C where you can actually find the Messengers on three islands and awaken them.