Root Of Bitterness

Strive for peace with everyone, and for the holiness without which no one will see the Lord. See to it that no one fails to obtain the grace of God; that no “root of bitterness” springs up and causes trouble, and by it many become defiled - Hebrews 12:14-15

What a "root of bitterness"? The first link is a list of Bible verses talking about bitterness and the second link is an explanation about what a root of bitterness is:

What is a root of bitterness?