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If you tap a heavy 50 kg50\text{ kg} refrigerator with your fingertip, it doesn't move because friction pushes back with that exact gentle force—not its maximum capacity.
Friction is an adjustable resistance force that matches your effort step-for-step until it hits a strict ceiling known as limiting friction.

The Normal Reaction Force

Whenever two solid objects touch, the contact surface exerts a perpendicular push called the normal reaction force (RR) that prevents the objects from passing through each other.
In 1699, Guillaume Amontons and later Charles-Augustin de Coulomb discovered that maximum friction between dry surfaces does not depend on the surface area of contact, but rather on how firmly the surfaces press together.
We quantify the roughness of the contact surfaces with the coefficient of friction (denoted by the Greek letter μ\mu), a unitless number where higher values represent rougher, stickier materials.
How does the friction force behave before and right at the exact moment an object starts to slide?

The F ≤ µR Model

For a stationary object, friction FF obeys the inequality F≤μRF \le \mu R, providing only the precise force needed to preserve equilibrium.
When an applied load increases to the verge of causing motion, friction reaches its maximum possible value Fmax=μRF_{\text{max}} = \mu R, a condition called limiting equilibrium.
What happens once the pulling force overcomes limiting friction and the body begins to slide?