What does this lever do near my gas heater?
It's a "damper". It can be manually set (open or closed) depending on quantity of air needed. 

During the design of the HVAC system, duct sizes are determined based on size and location of rooms they serve...kind of a best guess. Once the system (ducts) are installed, the quantity of air can be adjusted by opening or closing these dampers. 

Often we put these dampers on "fresh air" intake ducts. However, they're still used to adjust the amount of outside air. (We usually start at 10% of the total air required.) In residential construction we usually don't have fresh air intake ducts, because of air leakage around doors and windows. 

This damper appears to be on the "return air" duct. Keeping the damper "full open", uses the maximum amount of return air.