Matthew 6:19-24

Treasures and possessions/money.
v.19-20 Don't store treasures for yourself on earth where moths will eat it, or rust corrode it away, or thieves steal it (i.e., don't be greedy). 'Rust' may indicate decay and destruction by mildew, rats and the like. Store your treasures in heaven (i.e., things done in this life that have eternal value). No one can steal treasures you have stored in heaven. Therefore, work for spiritual riches like: holiness of character, obedience to God's commandments, souls won for Christ, disciples nurtured in the faith, and compassionate use of material resources.
v.21 Where your treasure is, there will also be your heart (mind, emotions, will). See Lk 12:33-34. Your heart will be drawn to what you place the highest value on. This does not imply rich people cannot be Christians, but only that riches bring grave dangers to the equation. Place the Father's kingdom first and your needs will be met (v.33). Love of wealth is a great evil (1 Tim 6:10).
v.22-23 Your eyes enable you to see. Good and bad eyes may refer to a good and a bad heart and therefore to storing treasures in heaven or on earth. See Lk 11:34-36. We need to maintain a clear perspective on where to put our treasures. The man with bad eyes, walking in darkness, thinks he has light. But this light is really darkness, made all the worse since he does not recognize it for what it is.
v.24 The disciple must serve God wholeheartedly in all areas of his life. You cannot have both treasure in heaven and on earth, you will end up choosing one or the other. You cannot serve both God (producing heavenly treasure) and Money (producing earthly treasure). See Lk 16:13.

Read Matthew 6:19-24