Skill Cards are temporary buffs that you can take advantage of during the duration of a Level.

Every time you finish a sector, you will be presented with a skill or booster selection screen.

   
You can choose one card out of the three to buff a specific unit in your army (you have the option to acquire all of the cards after watching an ad).

These cards provide a temporary buff that will only apply for the duration of the level.

Once a level is finished, units will return to their normal (un-buff) state.