The good explanations you received lack a fundamental definition that will help those without formal CS education or some experience in FP to understand that code :

A lambda is a function that take two arguments:

- a list of arguments A

- a block of code C

And return an anonymous function having the arguments in A and C as a body. In python lambda is an expression and the syntax is :

  #A is the part before the : and C the part after
  lambda a1 a2 a3 : a1 + a2 + a3
  

A more formal definition is available in the Racket guide https://docs.racket-lang.org/guide/syntax-overview.html#%28p...