Quick answer
Religious performance without a life behind it repels God; the plain prayer of an honest person delights Him. The costly offering loses to the free one.
Read both lines
The comparison is engineered to shock an ancient reader. A sacrifice was expensive, public, and unmistakably religious; a prayer cost nothing and could be whispered. The verse reverses the obvious ranking, and Samuel had already said the same thing to a king who kept the ceremony and lost the point: "to obey is better than sacrifice" (1 Samuel 15:22). What God is rejecting is not worship but worship used as cover, ritual offered by someone with no intention of changing, which turns the offering into a transaction and God into someone who can be handled. The second line is the warmer half and the one people skip. The prayer of the upright is His "delight," His pleasure, which says that God is not merely tolerating honest people. He enjoys hearing from them.
From this side of the cross
The believer never has to wonder which category their prayer falls into, because the standing was settled by Another's offering rather than by the quality of theirs. Approach is not earned each morning by the sincerity of the approach.
Related: Proverbs 15:29 · 1 Samuel 15:22 · Proverbs 21:3
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.