The descriptions of lexical analysis & syntax use a modified BNF grammar notation.This uses the following style of definition

Question

name ::= lc_letter (lc_letter | "_")*
lc_letter ::= "a"..."z"
I couldn't get it.

Answer 1

this operator "do" nothing...
at left there's the token name, at right the token description

basic token have simple description, more complexe token could chain tokens: that's a kind of dictionary of vocabulary/grammar used in a (programing) language...

it's used to build parser/analyzer of languages ;)

Answer 2

you should read it from bottom to top:

lc_letter := "a"..."z"

lc_letter represent range of "a" to "z" char

name := lc_letter (lc_letter | "_")*

name represent one lc_letter folliowed by any number (*) of lc_letter or "_" char...

Answer 3

"abc_def" is a valid 'name' token
"_abc" is not a valid 'name' token

Answer 4

::= or := what does this operator do ? that's why I am not getting.