Compressive strength of concrete means the concrete cubes are made in desired mix of concrete size is 150X150X150mm. Then the cube is curing (the cube is submerged in water) in 28 days. After 28 days the cube is test in compressive strength machine. The compressive strength value must be equal to design ratio value.
Example: If you are testing M20 concrete the compressive strength value is 20 N\mm2.