Can you explain lambda in lisp?

Question

Lisp

I'm starting to understand Lisp a little (I want to attach one thing to emacs). And I don't understand the tutorial:

...
(let* ( (...)
  (lines (...) ) 
   )
  (insert 
    (mapconcat '(lambda (s) (concat "<key>" s ) )
    lines ;; как передается переменная, которая ниже в параметр lambda?
    "\n"
  ) )

Actually, the question is in the comment itself - I don’t understand by what law lines was passed to the lambda parameter?
How does is called?
Thanks to all.

Answer 1

The mapconcat function calls the function passed in the first argument (lambda in your case) for each element of the sequence passed in the second argument, and the returned values ​​are glued into one line, separated by the third argument.

(mapconcat 
  '(lambda (s) (concat "<key>" s ))
  lines
  "\n")

That is, if linesit contains '("a" "b" "c"), then first the string "a" will be passed to the lambda, there it will be glued with the string "<key>", then the string "b", then "c", and after the received strings "<key>a", "<key>b" and "<key>c" will be concatenated into one string "<key>a\n<key>b\n<key>c".