the sheilds reduce the damage your ship recieves from the attacking stack of ships, depending on what type of attack they have
for example: the attacking ship has 100 energy attack, and your defending ship has a energy sheild of -50%, the attacking ship then fires on your ship, and your ship will receive 150 damage. this is because 100+(100 x .50)=150
but what if the your ships sheild is positive, like, some missle ship has 1000 missle attack, and fires on your ship with a 25% ECM (missle sheild)
then your ship will receive 1000 - (1000 x .25)= 750 damage
now the attack types pretain to certain sheilds
energy-energy sheild
kinetic-absorbtion sheild
missle-ECM
chemical-ionized hull
so if you are attacked by a ship with kinetic, and you have +95% ECM, that sheild will not play into that battle (unless later attacked by a ship with missles)
also, all the things on the ships stats should be blue with question makrs beside them, that way you can click on them and they will offer an explanation