Letters on a gas shut off/on valve meaning
P is probably "pilot" for a standing pilot light - L is presumably for "Lit" or on. Typically you'll have off, pilot, and on for many gas appliances old enough to have a standing pilot light. Newer ones are prone to use some other sort of ignition for better efficiency when "off".

Depending on other letters or positions not mentioned, L might also be for "light" as in start the full flame from the pilot at this position - if it can turn up from there, you don't want to go past there until it is lit.