I am kind of flummoxed by the opening sentence: "mass, time, and length can be converted into one another as the universe evolves." Later on, it goes on "Mass

I am kind of flummoxed by the opening sentence: "mass, time, and length can be converted into one another as the universe evolves." Later on, it goes on "Mass and length are also interchangeable... Basically, as the universe expands, time is converted into space, and mass is converted into length." How do you convert length into mass? The length of what?