Matthew 6

I sense that in the Savior's Sermon on the Mount, some of the counsel was given for the masses or the multitude, whil other counsel was given specificially to his disciples or ministers. Later in the sermon, the Savior instructs -

"Lay not up for yourselves treasures upon earth, where moth and rust doth corrupt, and where thieves break through and steal:
20 But lay up for yourselves treasures in heaven, where neither moth nor rust doth corrupt, and where thieves do not break through nor steal:
21 For where your treasure is, there will your heart be also." (New Testament | Matthew 6:19 - 21)

I sense at this time He is specifically addressing His disciples in that it are these that He will send out to teach and testify. James E. Talmage wrote -

"Treasures in heaven" could have two meanings: (1) the things we must do to inherit heaven, and (2) the things we do or acquire on this earth that can be taken with us into heaven.

One lays up treasures in heaven when he is baptized, receives the gift of the Holy Ghost, and remains faithful to the end. Knowledge and righteous attributes (honesty, dependability, mercy, love, etc.) are also "treasures in heaven," for we have been told, "Whatever principle of intelligence we attain unto in this life, it will rise with us in the resurrection." (DC 130:18.)

Many there were and many there are whose principal effort in life has been that of amassing treasures of earth, the mere possession of which entails responsibility, care, and disturbing anxiety. Some kinds of wealth are endangered by the ravages of moths, such as silks and velvets, satins and furs; some are destroyed by corrosion and rust—silver and copper and steel; while these and others are not infrequently made the booty of thieves. Infinitely more precious are the treasures of a life well spent, and wealth of good deeds, the account of which is kept in heaven, where the riches of righteous achievement are safe from moth, rust, and robbers. Then followed the trenchant lesson: "For where your treasure is, there will your heart be also." (JTC, p. 242.)

And with teaching "laying up treasures", the Lord teaches His disciples -

"The light of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light." (New Testament | Matthew 6:22)

Making the verse more clear, the Prophet Joseph Smith added -

JST Matt. 6:22 ... single to the glory of God ...; (New Testament:Matthew 6:22)

...suggesting that laying up treasures is having an eye single to the Glory of God.