The Wholesale Price Index — tracks price changes at the wholesale/producer level, ahead of consumer prices.
WPI measures inflation at the wholesale level — prices producers and traders pay each other — rather than what consumers pay at the till (that's CPI). Because wholesale price changes often feed through to consumer prices with a lag, WPI is watched as an early, leading signal for where CPI inflation may be headed, though the RBI's actual policy target is CPI, not WPI.