Question from a newbie about After market

I'm very new at trading but do know about pre-market and after market hours.

What i dont understand is how it works? Take for example GNMK, a stock i am currently long in.

Just before closing, on my level 2 i see about 14k shares selling for $5.15. It then closes, but during after market hours someone buys 100 shares at 4.96, dropping it way down.

What the hell? How does this effect the opening bell tomorrow? will it open at 4.96 if there are no other trades until then? will it correct back up?