"Cabin Pressurization" refers to the process of increasing the air pressure in airplane cabins about ambient. In and before WW-2, most planes didn't have this capability, so they were limited to...
At the altitudes where commercial airliners fly, the atmospheric pressure is too low and people can't stand it (the gases inside you humans would expand), so you need pressurization.Also (us humans)...
If your talking about commercial jets, the cabin pressure is equivalent to the pressure at 8,000ft AMSL when at cruising altitude. Max differential pressure is around 7 PSI.