Sorry about the military jargon. PSYOP stands for "psychological operations." It means putting out messages (often with a hostile intention behind them) to influence the course of a war.
They do leaflets, loudspeaker announcements, posters, radio and TV broadcasts (from specially outfitted Air Force aircraft) and lots more. In Bosnia, they do comic books to help warn little kids of the dangers of land mine.
In the Iraq invasion, they encouraged Iraqi troops to surrender.
When all else failed, they used the massive loudspeakers on their Humvees to blast heavy-metal music out to US combat troops preparing for battle.