public display affection means kissing hugging in front of others
but from a dictionary it's another meaning
•S: personal digital assistant, PDA, personal organizer, personal organiser, organizer, organiser (a lightweight consumer electronic device that looks like a hand-held computer but instead performs specific tasks; can serve as a diary or a personal database or a telephone or an alarm clock etc.)
it is a Public Display of Affection . Couples usually do it to show how they care for each other. sometimes it goes a bit over board like groping , and sucking each others face (making out) there is good PDA and there is gross PDA.