Honda HRB rotary lawnmowers run on regular unleaded gasoline, so when fuel mixture is discussed, that refers to the proper fuel-to-air mixture. You adjust this mixture with the mower's throttle control, since the carburettor on this particular mower can't be adjusted. The fuel mixture is adjusted depending on whether the engine is hot or cold. Honda has made this a simple process for the user.
Check the fuel level in the gas tank. If the level is too low, the mower will stall from lack of fuel. It's also good practice to top off the tank before you start mowing.
Move the throttle level to the choke position if the engine is cold. This makes the fuel mixture richer -- that is, the proportion of fuel to air is greater.
Move the throttle to the fast setting once the mower is running smoothly. This makes the fuel-air mixture leaner by reducing the proportion of fuel. Use this setting if you're starting an engine that has been running recently.