>>I understand that at the command "while (isspace(ch=getc(in)))" the programm reads till if founds a character that is either a " or a blank.
You understand incorrectly. isspace returns TRUE if the character given (which is the one returned by getc and assigned to ch) is a whitespace character, of which there are several: space obviously, but also tab, line feed, carriage return and possibly others. To find out exactly, run something like:
for (int i=0; i<256; i++)
if (isprint(i) && isspace(i))
printf("'%c' is a space\n",i);
So the command will loop until ch ISN'T a whitespace character.
>>Is it then correct to say that the program starts again at the same position as before
No, getc returns the next character from the file each time it is called, so the first getc after the above while will return the first character after the delimiter. What your code will do, for example, is to read input like " @foo@" and write "foo" to str.