What the 'gear on the left handlebar' does... is this a joke post? Trolling a bike forum with such questions?!? On the left you have the front derailleur and front brake typically, configurations may vary. Hmm...
Well I will be a fool and assume this is a genuine post. The shifter on the left moves the chain on the front derailleur between the front 2 (or3) chainrings. thus putting you in the higher (or lower) gear range, the higher gear range being faster (ie:harder to pedal) and the lower gear range being slower (or easier to pedal) making it easier for climbing.
I do, however find it hard to fathom that you have been riding for 2 years and did not know this, or is this your first road bike?