also, could someone explain shield mitigation?
Shield mitigation is the game's way of reducing incoming damage when a unit is being focus-fired on so it doesn't die as quickly.
It is essentially a direct damage reduction. Shield mitigation is only present on units that have shields (even if the shield hp is currently at 0, damage is still mitigated).
Mitigation starts at 15% base, which means right off the bat the ship takes 85% of the damage fired at it. Each point of damage taken increases the ship mitigation value by a small amount, and at the same time each second that passes decreases the current mitigation by a larger amount. Basically, the ship has to continuously be taking a fair amount of damage for its mitigation to increase. When it's not taking damage, the shield mitigation drops but never goes below the base. The maximum mitigation varies, but is usually 60% and up (Advent get more through their culture research and shield research).
It's a very powerful game mechanic, especially once starbases come into play that have 10k+ shields and 15k+ hull hp. For example, going from 60% mitigation to 70% (Advent culture/shield research) is a 25% damage reduction: at 60% the unit takes 40 damage out of 100, at 70% it takes 30 out of 100.
Hope this clarifies it a bit. 