Quick answer
Still water shows you your own face. The verse says a person's heart does the same thing, and the comparison is about accuracy rather than about insight into others.
Read both lines
The Hebrew is spare to the point of being cryptic: literally "as water, the face to the face, so the heart of man to man." There is no verb in either line, which is why the NKJV supplies "reflects" and "reveals," and other versions supply differently. Some read it as self-knowledge, water showing you yourself; others read it as one person recognizing another, the way you see your own humanity in someone else. Both readings live inside the same six words.
The catalog does not settle it, and the reason is that the verse is stronger for holding both. Water only reflects when it is still, which is the detail neither reading can do without. Whether you are looking at yourself or at another person, agitation ruins the image, and the verse describes the one condition under which anything can be seen accurately.
Related: Proverbs 20:5 · Proverbs 21:2 · Proverbs 16:2 · Proverbs 27:17
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.