What does Numbers 14:18 mean?

You said, The Lord is slow to anger and full of loyal love, forgiving wrongdoing and rebellion. But he doesn't let the guilty go unpunished—he brings the consequences of parents' sins on their children, even to the third and fourth generation. Numbers 14:18 - Modern Text Bible

(Moses quotes God's own words about his character: patient, loving, forgiving, but also just.)