The fear of the LORD is the beginning of knowledge: but fools despise wisdom and instruction.
The fear of the Lord (is) the beginning of knowledge - the grand summary of the whole book (cf. Psalms 111:10, "The fear of the Lord is the beginning of wisdom"). Solomon begins his lessons on true wisdom by laying down the first principle and basis of it, which is "the fear of the Lord." This means reverent trust, love, and obedience toward Him. Such reverent fear presupposes the knowledge of how infinite in power, majesty, and goodness God is. He who thus fears God will fear to sin, lest he should displease God (cf. Job 28:28, which was before Solomon's eye in this verse).
(But) fools despise wisdom and instruction. "Fools" are those who "know not God," and who are "wise to do evil, but to do good have no knowledge" (Jeremiah 4:22). The sum and subject of the first nine chapters is primarily the fear of the Lord the true knowledge; and secondarily, wisdom and instruction - i:e., disciplining the life in consonance with the fear of the Lord, from which these two spring.