A digital computer is a symbol manipulator, a symbol in this sense is a discrete letter or digit.

A computer spell checker has no intelligence, it just manipulates symbols, blindly, robotically.

All software is the same - symbol manipulation - be it simple software or complex software its just symbol manipulation.

Another way to answer you is that a symbol is what digital computers manipulate.

A digital computer is a Turing machine and a Turing machine is one of several ways of defining “computation”.

Take addition, lets devise a system for adding single digit numbers.

Well the base case is 0 + 0 = 0, then we have 0 + 1 = 1 and 1 + 0 = 1 and so on.

Addition is just a table lookup, if we have a 2D matrix with 0 - 9 along the bottom and 0 - 9 running vertically then we can populate the table so that whenever we lookup X,Y we get a value that is X + Y, that’s it, that easy, we can add any single digit number yet there’s no intellect involved, just a mechanical system.

The point is all software even the most complex, always boils down to this robotic, mindless type of mechanism and if adding two single digit numbers is mindless then so is all the other stuff like calculating square roots, drawing mandelbrot set images, performing special effects, speech recognition and so on, in every single case - bar none - all that’s going on are simple step by step operations nothing more.