What are these icons?

[Tinjaw]

Screenshot http://screencast.com/t/3c2z4UzvJd

What are the icons that look like shields? I don't have them on my units, but the enemy does. I cannot determine what it means or how to get it.

Any help would be appreciated.

[tiefenrausch]

Those little shields indicate the level of unit upgrades. Beside the general upgrades for all units (health, armor, speed...) which you do in the barracks (b-button), you can also upgrade a specific type of units.
You can upgrade each unit three times (bronze, silver, gold). The color of the shield icon shows the upgrade level of the unit. These upgrades are more efficient than the general upgrades and therefore should be bought preferantially, so after unlocking new units you want them to be on gold as soon as possible.
Unit upgrades are done at the same screen where you unlock units (accessible by pressing the u-button on your keyboard). Just click on an unlocked unit again.

[tiefenrausch]

btw, if you want some inspiration on upgrade order and tactics in general, I recommend you to watch the videos I posted in this thread:
http://steamcommunity.com/app/214360/discussions/1/540732888710380102/

[Tinjaw]

@tiefenrausch

Thank you. That is very helpful. I will be sure to check out your video. Thanks.