The Greek adjective kruptos means hidden, secret, or concealed. It is the root of the English 'cryptic' and describes what is not openly visible — whether hidden sin, secret motives, or the hidden person of the heart.
Jesus uses kruptos both to warn against religious hypocrisy (performing acts to be seen) and to promise reward for hidden faithfulness: 'Your Father, who sees what is done in secret (kruptos), will reward you' (Matthew 6:4, 6, 18). Paul uses it for the 'hidden person of the heart' (1 Peter 3:4) and for the day when God will judge the hidden things of men (Romans 2:16).
Theologically, kruptos teaches that God is the God of the hidden — He sees what no one else sees, rewards what no one else knows, and will expose what has been concealed. Authentic faith is practiced before God's eyes alone.