This pic shows what happens during 5 combustion strokes in a row (5 solid lines) inside the same cylinder.
The X-axis on the bottom is crankshaft degrees where zero degrees = TDC (when the piston is at the very top of it's travel and finished squeezing or compressing the air-fuel mix on the compression stroke).
Any crank degree to the left of zero (negative number) is when the piston is still squeezing the air-fuel mix on the compression stroke. This is called Before TDC or BTDC. So at - degrees BTDC means the piston is squeezing and pushing up. By -10 degrees BTDC, as we move to the right on the graph's X-axis, the piston has squeezed some more and is higher up in terms of it's position and it's nearing the top of it's travel.
Any crank degree to the right of zero (positive number) is the power stroke. This is called After TDC or ATDC. The piston is now being pushed back down by the combustion explosion and turning the crank. So at +10 degrees, the piston has dropped down. By +20 degrees , as we move rightward along the X-axis, the piston has gone down even more, away from the very top (TDC) and has turned the crank further.
The dotted line on the very bottom shows you what the cylinder pressure would be if we did NOT light (ignite) the air-fuel mix with a spark. This is what you measure when you do a compression test. It's also called the "cranking pressure".
Also notice that when you add a spark and cause an explosion (the 5 solid lines), the cylinder pressures shoot up way higher than compared to when the piston just squeezes the air-fuel mix with no spark (the dotted line).