What is the function of the left foot lever on a motorcycle?

The left foot operates the gear shifter on a standard motorcycle, using the standard '1-down, rest-up' pattern: first gear is down (toe pressed down), neutral is between first and second (a half-click up from first), second through sixth are progressive clicks up. The right foot operates the rear brake. The hands operate the clutch (left lever) and front brake (right lever). The standard 1-N-2-3-4-5-6 shift pattern is essentially universal on modern motorcycles, including scooters with manual transmissions, sport bikes, cruisers, and adventure bikes. Some older or specialty motorcycles use different patterns, but new riders should expect the standard.