When two people, who, by chance like each other, but can't commit themselves to a relationshipstraight away (maybe due to age difference, or other situations etc.) agree to play The Long Game, essentially saying "I'll wait for you".
Susan knew that all she had to do was wait and watch, and eventually her cruel boss would do something so bad she'd be able to report him. She was good at playing the long game.
His friends called it being friend zoned, he called it playing the long game, but she called it being a fuckboy.
On the show Supernatural, both Big Bads have been playing Long Games to fulfill their plans. We won't find out what the final result will be until the season finale.