So, cylinder pressure is the constantly changing pressure inside the cylinder for all 4 strokes. There are certain characteristics of this pressure which people may want identified, like peak (maximum) cylinder pressure near TDC, or peak cranking compression pressure (pressure at TDC without combustion), or average cylinder pressure (IMEP, indicated mean effective pressure). They are all just measures of cylinder pressure at certain times (or averaged over certain times) in the 4 strokes.
It should be noted that for 3 of the 4 strokes, the engine is putting work into the process, and that only during Combustion/Expansion stroke is any work being output from the process. The work during the Combustion/Expansion stroke must make up for the other 3 strokes and still have enough left over to produce a net power. Luckily it works out that there is LOTS of work (or energy released) produced during Combustion/Expansion stroke.