not sure that you could explain it any clearer than what Joe stated but...

a compression tester will only tell you the PRESSURE that is being built up in each cylinder. .for example if you were to test each cylinder with a compression tester and it looked like this:
1 - 80psi
2 - 75psi
3 - 82psi
4 - 40psi
5 - 77psi
6 - 76psi
7 - 74psi
8 - 73psi

that would only tell me that something is wrong with cylinder 4 (rings going bad, bent valve, etc).. and I would need to investigate further..