PV diagram

Pressure up the vertical axis, volume along the horizontal one. Essential knowledge 9.4.B.2 calls them pressure-volume graphs, also known as PV diagrams, and describes them as representations of thermodynamic processes.

A point is a state, a curve is a process. Fixing $P$ and $V$ fixes the temperature through $PV = nRT$, and for a monatomic ideal gas it fixes the internal energy as well, since $U = \tfrac{3}{2}nRT = \tfrac{3}{2}PV$. The same point always carries the same $U$, whatever route the gas took to reach it.

Area is work. 9.4.B.2.ii puts it carefully: the absolute value of the work done on a gas as it expands or compresses equals the area underneath the curve on a plot of pressure against volume. An area is a magnitude and carries no sign, so the sign comes from the direction of travel together with the sheet's $W = -P\Delta V$, work done on the gas. Move right, the gas expands and $W$ is negative. Move left, it is compressed and $W$ is positive. Move straight up or down and the region has no width, so $W = 0$.

A closed loop encloses the net work in magnitude, because the areas under the outward and return legs overlap and cancel. See cyclic process.

9.4.B.2.i supplies the vocabulary for the curves: a line of constant temperature on a PV diagram is an isotherm.

Reading a particular diagram, assigning signs leg by leg and totalling a cycle is the PV diagrams guide's job.