Three years ago the bubble tea you drank all the time cost 15 yuan; today the same cup is 18 yuan. Same cup, more money out of your pocket.
Look at it another way: back then 100 yuan bought about 6 cups, now it only buys a little over 5. You don't have less money—your money just buys less.
This ability—"how much stuff the same money can buy"—is called purchasing power. When prices rise across the board, purchasing power falls.