Show people a blob and a star and ask which is bouba: around 95 percent agree, across ages and languages. Wolfgang Kohler ran the original version in 1929 with maluma and takete; the modern naming comes from Ramachandran and Hubbard.
The effect matters because it is sound symbolism measurable in nearly everyone, a shared crumb of the cross-sensory mapping synesthetes have in full. It also suggests constraints on how words attach to things, with round vowels fitting round objects.
In visual design the effect is load-bearing: audiences expect harsh transients to look sharp and warm pads to look soft, and designs that violate the mapping read as wrong before anyone can say why.