Hi bp. E-night is a night when 9 specific attractions in the MK are open for an additional 3 hours after closing. It does cost extra (12 per adult plus tax). You must be a resort guest and must have a multi day pass (park hopper, AP, LOS, etc). The ticket is only for e-night. It doesn't gain you entrance to MK. So if you are using hoppers you will have to use a day off of it. But of course if you were in a park earlier in the day, then you're just hopping so it's no different then if MK were open for the evening and you were hopping over.
I don't think you necessarily have to worry about avoiding MK on days when there will be an e-night. I know when I go to e-night I usually go somewhere else during the day because I'm going to hit all my favorite rides at MK that night.