For the moment all discipline seems painful rather than pleasant, but later it yields the peaceful fruit of righteousness to those who have been trained by it. ~ Hebrews 12:11 (ESV)
The author is speaking here about the fatherly discipline of God in the lives of believers. Discipline is different from punishment. Punishment seeks to exact retribution now for a wrong done in the past. Discipline, by contrast, seeks to instill a change now that leads to a better outcome for the future. If we yield to the short-term unpleasantness of the discipline of God today, we can expect to taste the sweet fruit of peaceful right-living tomorrow.