Y/Y stands for "year-over-year."
It's a comparison used to evaluate the rate of change in statistics, interest rates or other financial performance measures.
If you have a CPI value of 200, for example, and one year later the CPI is 210, that is a 5% y/y increase.
Y/Y measures eliminate the effects of seasonality because they compare similar periods of time (e.g., this month versus the same month last year).