It means the power levels they are actually at. Hyper-impulse engines are best if all your skills and buffs put your engine power over 50, even if you still have it preset at 25. Drop those combat impulse engines and get hyper-impulse engines if you can.
Hyper impulse engines are also very handy since putting a large amount of power into them can get you some very nice impulse speeds (40+) for the times you need to get from point A to B and you can't drop out of combat
combat engines are speedier at low power levels (<40), but get minimal gain when increasing power and have the lowest top speed of all.
"normal" engines have a flat increase in speed according to how much power you give them. average, average, average.
hyper engines are speedier at high power levels (>60) and so the speediest in the game, but take a heavy speed hit if power is low.
Make your choice depending on how much engine power you normally have in system space. With all the bonuses added.