The phrase indicates a complete switch in roles or situations, often with a sense of irony He used to be the coach, but the shoe is on the other foot now and he's the player.
Pro Tip 2/3
Common Reveal
It's often used to reveal a change in dynamics that might be surprising or unexpected She always won our games, but the shoe is on the other foot this time.
Pro Tip 3/3
Not Literal
Remember, this phrase is metaphorical and never refers to actual footwear He laughed last year, but with the shoe on the other foot, he's not laughing now.