The part of the condition-case expression that is evaluated in
the expectation that all goes well has a when. The code uses
when to determine whether the string variable points to
text that exists.
A when expression is simply a programmers’ convenience. It is
like an if without the possibility of an else clause. In your
mind, you can replace when with if and understand what
goes on. That is what the Lisp interpreter does.
Technically speaking, when is a Lisp macro. A Lisp macro
enables you to define new control constructs and other language
features. It tells the interpreter how to compute another Lisp
expression which will in turn compute the value. In this case, the
other expression is an if expression.
The kill-region function definition also has an unless
macro; it is the opposite of when. The unless macro is
like an if except that it has no then-clause, and it supplies
an implicit nil for that.
For more about Lisp macros, see Macros in The GNU Emacs Lisp Reference Manual. The C programming language also provides macros. These are different, but also useful.
Regarding the when macro, in the condition-case
expression, when the string has content, then another conditional
expression is executed. This is an if with both a then-part
and an else-part.
(if (eq last-command 'kill-region)
(kill-append string (< end beg) yank-handler)
(kill-new string nil yank-handler))
The then-part is evaluated if the previous command was another call to
kill-region; if not, the else-part is evaluated.
yank-handler is an optional argument to kill-region that
tells the kill-append and kill-new functions how deal
with properties added to the text, such as bold or italics.
last-command is a variable that comes with Emacs that we have
not seen before. Normally, whenever a function is executed, Emacs
sets the value of last-command to the previous command.
In this segment of the definition, the if expression checks
whether the previous command was kill-region. If it was,
(kill-append string (< end beg) yank-handler)
concatenates a copy of the newly clipped text to the just previously clipped text in the kill ring.