U+1F923 · emoji since 2016
This is laughter that knocked you over. The face is literally tipped sideways with tears flying, which makes it the step beyond 😂 when something is catastrophically funny.
People often stack it for emphasis or alternate it with 😂 to keep the laughter texture varied. If a joke gets 🤣🤣🤣, the sender wants you to know they made a scene.