Friction force comes from this equation, F = μN, where F is the force, μ is the coefficient of friction between surfaces and N is the normal force of the ground on the bike. The normal force shouldn't change as Rahul rides, as it's dependent mostly on he and his bike's weight. What's changing is the coefficient of friction, μ, which in general is a higher value for static friction (not moving) than it is for moving friction (sliding (kinetic friction) or rolling). Basically, it takes more force to put something into motion than it takes to keep it in motion once it is already going. That's why Rahul has to push harder on his pedals to start going than he goes after he's already rolling along.