Forth is a high level programming language which operates similarly to an RPN (Reverse Polish Notation or postfix) calculator. It differs from typical programming languages like C and Fortran because it is an interactive language. If you are using Forth, you do not need to recompile your program to add some new functionality. You can easily define a new command and it will instantly be available for you to use.
move to filter j, take a source image move to filter h, take a source image move to filter k, take a source image start overtyping each command over and over. Since you know you are going to do the same thing repeatedly, you might as well make your job easier (don't worry about what the symbols mean yet):
: jhksrun ( -- ) \ Take images at j, h and k filter wavelengths. j srun h srun k srun ;Now you need only type jhksrun to get images at three different filter wavelengths. If you had used the C language instead of Forth, you would have had to modify the source code and recompile the program, losing valuable time. Without leaving Forth you defined a word and used it with no time wasted.