Plain English Meaning
It's how you steer the plane on the ground. You push the pedals with your feet to turn the front wheel left or right, the same way a steering wheel turns a car.
Full Definition
A ground-handling system that allows the pilot to turn the airplane during taxi by steering the nosewheel directly through the rudder pedals. On most light airplanes the rudder pedals are mechanically linked to the nose gear, so pressing the left or right pedal turns the nosewheel in that direction. On larger aircraft a separate hand-operated tiller may also be used for tighter turns.
Context Anchor
You encounter nosewheel steering during taxi, especially when using the rudder pedals to keep the airplane moving along the taxiway centerline.