please explain the answers in green in the image

Consider a language based on the lambda calculus that includes function definition, function application,

numbers, and arithmetic operations $($addition$,$ subtraction, multiplication, division$)$:

$($a$)$ Write a context$-$free grammar for this language.

$($b$)$ Write a variant for the abstract syntax of this language.

type expr larr

$|$ Num of number

$|$ Var of string

$|$ FuncExpr of string $$ expr

$|$ CallExpr of expr $$ expr

$|$ BopExpr of expr $$ bop $$ expr