What does the root of bitterness really means?

What is the root of bitterness from bible perspective?

looking carefully lest anyone fall short of the grace of God; lest any ROOT OF BITTERNESS springing up cause trouble, and by this many become defiled;
Hebrews 12:15 NKJV

so that there may not be among you man or woman or family or tribe, whose heart turns away today from the Lord our God, to go and serve the gods of these nations, and that there may not be among you a ROOT BEARING BITTERNESS OR WORMWOOD; and so it may not happen, when he hears the words of this curse, that he blesses himself in his heart, saying, ‘I SHALL HAVE PEACE, EVEN THOUGH I FOLLOW THE DICTATES OF MY HEART’— as though the drunkard could be included with the sober.
Deuteronomy 29:18-19 NKJV

The root of bitterness has nothing to do with feeling angry or bitter about someone or your own life. It is about hearts turning away from God. It is about thinking you are ok, but you are really not. Like the Church of Laodicea, who thinks they are well, but are actually naked and wretched.

Hebrews is about keeping the faith, by running the race.

Therefore strengthen the hands which hang down, and the feeble knees, and make straight paths for your feet, so that what is lame may not be dislocated, but rather be healed.
Hebrews 12:12-13 NKJV

Strengthen the weak hands, And make firm the feeble knees. Say to those who are fearful-hearted, “Be strong, do not fear! Behold, your God will come with vengeance, With the recompense of God; He will come and save you.”
Isaiah 35:3-4 NKJV