I have a 15 speed bike. What does the left speed changer mean?

The left gear shift moves the chain between the two cogs in the front. L means low, H means high, referring to speed. When the gear shift is on L, the chain is on the smaller cog, and the bike is easier to pedal. When the gear shift is on H, the chain is on the larger cog, and it is more difficult to pedal.