The combat rating (introduced in 2.4000) of a ship is a ship's effectiveness at destroying another ship, as well as it's ability to resist being destroyed itself. When two ships engage, the combat of the opposing ship is subtracted from the combat of this ship. If either ship is reduced to 0 or below, it is destroyed. If either ship still has combat remaining, then it may continue on to the next battle, or complete its mission depending on the type of ship.
Example Combat Scenario
As the first snark approaches, it is immediately destroyed by a drone. This leaves the drone with 5000 combat, however, the way drones deliver their 7000 combat is by crashing into their opponent, therefore the drone is destroyed as well.
The fourth snark gets through because there are no more defenders remaining.
However, the fifth snark gets taken out by the fighter, because the fighter has run through its reset time. Unfortunately, the fighter has depleted its combat taking out the snark and is therefore destroyed.
If an attacker gets through all the defenders with even 1 combat remaining, then it will carry out its mission. In the case of a snark, that would be to damage a building.