What does Matthew 6:21 mean?

Matthew 6:21 KJV
For where your treasure is, there will your heart be also.

Matthew 6:21 MKJV
For where your treasure is, there will your heart be also.

Matthew 6:21 NKJV
“For where your treasure is, there your heart will be also.

Matthew 6:21 KJV 2000
For where your treasure is, there will your heart be also.

Interlinear KJV

For /gar/ where /hopou/ your /humon/ treasure /thesauros/ is, /esti/ there /ekei/ will /esomai/ your /humon/ heart /kardia/ be /esomai/ also. /kai/

Albert Barnes’ Commentary

21. For where your treasure is–that which ye value most.

there will your heart be also–“Thy treasure–thy heart” is probably the true reading here: “your,” in Lu 12:34, from which it seems to have come in here. Obvious though this maxim be, by what multitudes who profess to bow to the teaching of Christ is it practically disregarded!  “What a man loves,” says LUTHER, quoted by THOLUCK, “that is his God.  For he carries it in his heart, he goes about with it night and day, he sleeps and wakes with it; be it what it may–wealth or pelf, pleasure or renown.” But because “laying up” is not in itself sinful, nay, in some cases enjoined (2Co 12:14), and honest industry and sagacious enterprise are usually rewarded with prosperity, many flatter themselves that all is right between them and God, while their closest attention, anxiety, zeal, and time are exhausted upon these earthly pursuits. To put this right, our Lord adds what follows, in which there is profound practical wisdom.