Bikes pedals are sometimes a challenge for people who aren't familiar with the mechanics of bikes. The right pedal is thread "normally", i.e. it has right-hand thread. The left pedal is the opposite.
I try to help people out by telling them to turn the wrench to the rear of the bike, over the top of the pedal. Or, you can think of it as turning the wrench in the same direction (lefty-loosey), if you turn both of them from the drive-side. But, still, people get confused and tighten the hell out of the left pedal.
So, why is one pedal right-hand thread, and the other pedal, left-hand?
Well, as you pedal along, the spindle of the pedal will try to turn itself toward the front of the bike. In the drive side, this has the effect of tightening the pedal into the crank-arm. On the non-drive side, it has the effect of unscrewing the pedal from the crank.
In the early days of bicycling, both pedals were threaded the same. This caused a problem with the left pedal backing out of the crank, which required the rider to stop and tighten the pedal rather frequently.
Eventually, someone figured out how to alleviate this problem (by threading the left pedal spindle with left-hand thread).
Who was this genius? I can't tell you for sure. But, I can narrow it down to one of two people: Orville or Wilbur Wright.
Yes, the Fathers of Aviation were, originally, bike mechanics and then, bike manufacturers. In the course of building bikes, they managed to solve a problem which had nagged cyclists for 30 years, when they figured out how to prevent the left pedal from unscrewing itself from the crank.
You have to wonder what else they would have come up with, bicycle-wise, if the airplane thing hadn't worked out...