What is the slap bet countdown?

The slap bet countdown is a humorous element from the TV show "How I Met Your Mother," specifically from the episode titled "Slap Bet." In this storyline, Marshall and Barney make a wager that results in Marshall earning the right to slap Barney a specified number of times over an undefined period. The countdown serves as a comedic reminder of the impending slaps, adding tension and anticipation to their interactions throughout the series. It underscores the show's blend of humor and ongoing character dynamics.