Friction charges by how hard the surfaces press together: $f_k = \mu_k N$ — read aloud f equals mu-k N, and $\mu$ is the Greek letter mu, said “mew”. Three letters, three jobs: $f_k$ is the friction force in newtons, $N$ is the normal force in newtons — the surface pushing back (on a level floor, exactly the weight) — and $\mu_k$ is the coefficient of friction, this course's first naked number: a force over a force, so every unit cancels. A coefficient carries none — if your μ ever wears newtons, a rearrangement upstream is lying to you.

The subscripts name the state, not a first-and-second: k is kinetic, already sliding, and s is static, still at rest. So two coefficients share the job: $\mu_s$ guards things at REST (the breakaway ceiling, $f_{s,\max} = \mu_s N$, the largest push the grip can still hold), and $\mu_k$ rules the slide after. Static is usually the stronger grip — which is why the hardest part of moving a couch is the first centimetre.