Quick questions on The trapezoidal rule for estimating areas and definite integrals, including the over- or under-estimate test

There are two ways the ordinates $y_0, \dots, y_n$ reach you.

Because the area under $y = f(x)$ from $a$ to $b$ is the definite integral $\int_a^b f(x)\,dx$ (for $f(x) \ge 0$), the trapezoidal rule is also a way to estimate an integral you cannot evaluate exactly. The phrasing "use the trapezoidal rule to estimate $\int_a^b f(x)\,dx$" is asking for precisely the same calculation as "estimate the area under the curve": list the ordinates, weight the ends once and the middles twice, multiply by $\frac{h}{2}$.

$h = \frac{b - a}{n}$, the spacing between adjacent $x$-values. Reading $h$ off a table means checking the $x$-values really are evenly spaced.