P&IDs stands for Piping and Instrumentation Diagrams.

Piping has to do with pipes inside a facility (Like refineries, Oil and gas fields, Power plants, Chemical plants etc).
NOTE: The term Piping is different from Pipeline. Piping are pipes inside a facility while pipelines are long distant lines outside the facility.

The Instruments measure four major variables namely:

Temperature
Pressure
Level and
Flow

For an instrument loop to be complete, there has be a

Sensor
Transmitter
Recorder/Comparer and
Control Valve carrying out the correcting function


To interpret P&ID symbols you need to be conversant with relevant
standard.

To learn more about Piping please follow the link below

http://everythingcad..com/2016/02/presents-normal-0-false-false-false-en.html