Under certain conditions, the solid and liquid states of water can exist in equilibrium. how are these conditions indicated on the phase diagram for water?

In a phase diagram that I have attached, the line marked in between ice and water (yellow and blue) is the conditions in which solid (ice) and liquid (Water) can coexist. As the graph shows, solid ice and liquid water coexist most when the temperature is zero, which should make sense since ice melts at 0, and water freezes as well at 0 degrees celsius. 

If you explain it more through i will answer it : )