1. The problem asks: What is friction? 2. Friction is a force that opposes the motion of an object when it is in contact with another object or surface. 3. It acts parallel to the surface and opposite to the direction of motion or attempted motion. 4. There are two main types: static friction (prevents motion when the object is stationary) and kinetic friction (acts during motion). 5. Friction arises due to microscopic roughness and interactions at the surfaces in contact. 6. The magnitude of friction is often modeled as $f = \mu N$ where $\mu$ is the coefficient of friction and $N$ is the normal force. 7. Understanding friction helps explain why objects slow down and how much force is needed to move or stop them.