The transmission of tighter monetary policy through to economic activity and inflation takes time. Monetary policy affects the spending and investment of businesses and households with a lag. In turn, those changes in demand take time to have their full effect on the setting of prices and wages.
These lags mean that central banks need to set monetary policy with a view to the future when it will be having its strongest effects. If instead the transmission of policy was rapid, we could use timely course corrections to navigate the economic path. However, the presence of lags in transmission adds a challenge to the setting of monetary policy.