AM - Ante Meridian, in Latin (Before Noon, in English)
PM - Post Meridian, in Latin (After Noon, in English)
AM and PM, came form the Latin words, which represents Before Noon and After Noon. They are used to refer to different times during day and night. It is frequently used in all types of Digital watches, mobiles and even in computer systems.
AM is used to represent time, from midnight (12:00) to noon (11:59) usually written as :- from 12:00 A.M to 11:59 A.M. In a sentence we can write - "I will reach college at 09:00 AM" it means, i will reach college at 9 o'clock in the morning.
Similarly, PM represents time, from noon (12:00) to just before midnight (11:59) usually written as :- from 12:00 P.M. to 11:59 P.M. In a sentence we can write - "I will reach home at 09:00 PM" it means, i will reach home at 9 o'clock in the night.