What is Normal force and friction?

Example 25.12 (The incline, twice)

A block slides down a slope of angle $\alpha$; axes: $x$ down the slope, $y$ perpendicular. On $y$: $N = mg\cos\alpha$. On $x$, frictionless: $ma = mg\sin\alpha$, so $a = g\sin\alpha$ — no mass; at $\alpha = 30^\circ$, $a = 4.9\,\mathrm{m}/\mathrm{s}^{2}$. With kinetic friction: $a = g(\sin\alpha - \mu_k\cos\alpha)$; for $\mu_k = 0.20$, $a = 9.81\,(0.500 - 0.173) = 3.2\,\mathrm{m}/\mathrm{s}^{2}$. And it stayed put in the first place only while the needed static friction fit under its ceiling: $\tan\alpha \leq \mu_s$.