The solid lines drawn on a phase diagram represent the equilibrium curves between two phases. If the temperature and pressure land exactly on a line, both phases can exist together perfectly. If you cross one of these boundary lines, the substance will change its physical state.
The specific spot where all three boundary lines meet is called the triple point. At this exact unique temperature and pressure, the solid, liquid, and gas phases all coexist. Another critical feature on the far top right of the graph is the critical point.
If you push the temperature and pressure past this dot, things get extremely weird. The liquid and gas phases blur together and become completely indistinguishable from each other. The substance transforms into a strange state known as a supercritical fluid.
These diagrams are incredibly useful tools for chemical engineers and materials scientists. They help workers predict exactly how a chemical will behave inside a pressurized factory pipe. Students often forget that ambient pressure plays a massive role in melting and boiling. A phase diagram proves that boiling water does not always require high heat.