As a very simple example of that, you use variables that behave like mathematical unknowns. When I run the following line of Mma code, x, y, and z are undefine

As a very simple example of that, you use variables that behave like mathematical unknowns. When I run the following line of Mma code, x, y, and z are undefined. It finds values for these unknowns. That's kind of different than many languages. Solve[{x + y + z == 3, y + x == 4, z + y == 7}, {x, y, z}] gives: {{x -> -4, y -> 8, z -> -1}}