6:14 What exactly is the meaning of this?

This means simply that like Paul, nothing should be as important to believers as the cross of Christ.

Because of the cross the world should be dead to them and they to the world.

The world should have no more appeal to them or influence over them.

The only important thing is that they become new creations in Christ (cp V15 with Ro 6:3-10; 2Cor 5:17; Ga 2:20; Php 3:3, 7-8, 20-21).

See also comments on Lk 15:14-15, Jn 15:18-25; Jas 4:1-4; 2Pe 3:1-7; 1Jn 2:15-17.