please explain the below c program

#include <stdio.h>
   #include <stdlib.h>  /* for  atof() */

   #define MAXOP   100  /* max size of operand or operator */
   #define NUMBER  '0'  /* signal that a number was found */

   int getop(char []);
   void push(double);
   double pop(void);

   /* reverse Polish calculator */
   main()
   {
       int type;
       double op2;
       char s[MAXOP];

       while ((type = getop(s)) != EOF) {
           switch (type) {
           case NUMBER:
               push(atof(s));
               break;
           case '+':
               push(pop() + pop());
               break;
           case '*':
               push(pop() * pop());
               break;
           case '-':
               op2 = pop();
               push(pop() - op2);
               break;
           case '/':
               op2 = pop();
               if (op2 != 0.0)
                   push(pop() / op2);
               else
                   printf("error: zero divisor\n");
               break;
           case '\n':
               printf("\t%.8g\n", pop());
               break;
           default:
               printf("error: unknown command %s\n", s);
               break;
           }
       }
       return 0;
   }

   int getop(char []);
   void push(double);
   double pop(void);

#define MAXVAL  100  /* maximum depth of val stack */

   int sp = 0;          /* next free stack position */
   double val[MAXVAL];  /* value stack */

   /* push:  push f onto value stack */
   void push(double f)
   {
       if (sp < MAXVAL)
           val[sp++] = f;
       else
           printf("error: stack full, can't push %g\n", f);
   }

   /* pop:  pop and return top value from stack */
   double pop(void)
   {
       if (sp > 0)
           return val[--sp];
       else {
           printf("error: stack empty\n");
           return 0.0;
       }
   }

#include <ctype.h>

   int getch(void);
   void ungetch(int);

   /* getop:  get next character or numeric operand */
   int getop(char s[])
   {
       int i, c;

       while ((s[0] = c = getch()) == ' ' || c == '\t')
           ;
       s[1] = '\0';
       if (!isdigit(c) && c != '.')
           return c;      /* not a number */
       i = 0;
       if (isdigit(c))    /* collect integer part */
           while (isdigit(s[++i] = c = getch()))
              ;
       if (c == '.')      /* collect fraction part */
           while (isdigit(s[++i] = c = getch()))
              ;
       s[i] = '\0';
       if (c != EOF)
           ungetch(c);
       return NUMBER;
   }