Interconversion of matter means that matter can de converted into energy and vice versa.This interconversion is governed by the Einstien's formula E=m*c^2 , where E is the energy, m is mass and c is the speed of light.This property of matter states that nothing can be destroyed completely in this world because when you destroy it, its some energy fragment survives. This even makes some of the scientists believe in the immortality of the universe