What does Matthew 6:18 mean?

Matthew 6:18 KJV
That thou appear not unto men to fast, but unto thy Father which is in secret: and thy Father, which seeth in secret, shall reward thee openly.

Matthew 6:18 MKJV
so that you do not appear to men to fast, but to your Father in secret. And your Father who sees in secret shall reward you openly.

Matthew 6:18 NKJV
“so that you do not appear to men to be fasting, but to your Father who [is] in the secret [place]; and your Father who sees in secret will reward you openly.

Matthew 6:18 KJV 2000
That you appear not unto men to fast, but unto your Father who is in secret: and your Father, who sees in secret, shall reward you openly.

Interlinear KJV

That /hopos/ thou appear /phaino/ not /me/ unto men /anthropos/ to fast, /nesteuo/ but /alla/ unto thy /sou/ Father /pater/ which /ho/ is in /en/ secret: /kruptos/ and /kai/ thy /sou/ Father, /pater/ which /ho/ seeth /blepo/ in /en/ secret, /kruptos/ shall reward /apodidomi/ thee /soi/ openly /en/. /phaneros/

Albert Barnes’ Commentary

Thy father which seeth in secret – Let us not be afraid that our hearts can be concealed from God; but let us fear lest he perceive them to be more desirous of the praise of men than they are of that glory which comes from Him.

Openly – Εν τω φανερω. These words are omitted by nine MSS. in uncial letters; and by more than one hundred others, by most of the versions, and by several of the primitive fathers. As it is supported by no adequate authority, Bengel, Wetstein, Griesbach, and others, have left it out of the text.