Yashar means straight, right, upright, just, or pleasing. It appears about 120 times. It describes both physical straightness and moral uprightness. 'Doing what is right in the eyes of the LORD' is a key evaluative phrase.
Uprightness is God's own character. The yashar person walks a straight path, avoiding the crooked ways of the wicked. Proverbs contrasts the straight path of the righteous with the twisted path of the fool, making moral straightness a core wisdom value.