When you are in-game arma3 Scenarios menu, you'll see all the mission names listed. When you click each of the mission names you get to see overviewPicture image, overviewText and author texts.

If you do not configure at least the texts properly, then your mission (or campaign) looks very poorly done.

Mission Description.ext:

overviewText = "The nuke is still out there!";
overviewTextLocked = "Finish the campaign in order to unlock this mission.";
overviewPicture = "overviewPicture_ca.paa";
author = "Snake Man, PMC.";

overviewPicture should be in PAA format and 1024 x 512 resolution.

See also campaign-description.ext