"Design Details" in Ship Designer

If you're talking about the row of icons at the bottom of the box, those are 'threat,' 'fortitude,' and 'value.' I don't know what these do, but I suspect it has something to do with targeting priority in combat and perhaps is a shorthand for the computer to use when evaluating whether or not to engage. These (or more specifically 'value') may also have something to do with how much the computer values the vessel in the trade screen.

If you're talking about the two rows of icons at the top, those are beam/missile/kinetic attack and defense (attack in the first row, defense in the second).

If you hover your mouse over the icons you should be getting a tooltip that tells you what the icons stand for.

If you are talking about the ones under the "Ship role" heading they are (Left to right) Threat, Fortitude, and Value.  All ship equipment (weapons, defences, drives, etc and not cosmetic parts) have at least one of these attributes assigned to them.  The figure in the design panel is simply a sum of these attributes of the installed equipment.  For example, lasers I think have a Threat of 0.2, so if you put 3 lasers on a ship your Threat in the panel will show 0.6.  If you hover over the number it will tell you where it is getting that number from as well.

These three attributes decide what the DEFAULT ship role is based on which of these attributes is the largest.  If Threat is the largest it makes the ship an attacker and if Fortitude is largest it makes it a defender.  Value has a special rule that it not only has to be the largest it has to be larger than the sum of Threat and Fortitude, if it is not it will go to the largest of Threat or Fortitude.  It also seems if Threat and Fortitude are equal it will favour Fortitude.