MDS / CPI
CPI - consumer price inflation
What it measures
Measures changes in consumer prices. Month-on-month compares with the preceding month, year-on-year with a year earlier.
Why traders follow it
Compare the actual with the forecast. Above-expected inflation may support the currency through expectations of higher interest rates. No country is specified here, so check the release. This is not a definition specific to US CPI.
This is a general definition. The country and calculation methodology matter for each specific release.