Automation systems are used to monitor and control a process to keep the process within a specified limit. The parameters that we wish to control at the correct limit are called Process Variables. A variable is something that can be changed over time. Because process variables can and do change, automation systems measure the variable and then control the variable to keep it within the given limits.
For example, we want to have air pressure value to 10 Bar for typical pneumatic system, If the pressure drops below 10 then we need to start the compressor. This 10 is a process variable which we control. Pressure, Temperature, Flow are the main three process variables in industries.