eintr: Prevent confusion
‘let’ Prevents Confusion
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The ‘let’ special form prevents confusion. ‘let’ creates a name for a
“local variable” that overshadows any use of the same name outside the
‘let’ expression. This is like understanding that whenever your host
refers to “the house”, he means his house, not yours. (Symbols used in
argument lists work the same way. 
The ‘defun’ Macro defun.)
Local variables created by a ‘let’ expression retain their value
_only_ within the ‘let’ expression itself (and within expressions called
within the ‘let’ expression); the local variables have no effect outside
the ‘let’ expression.
Another way to think about ‘let’ is that it is like a ‘setq’ that is
temporary and local. The values set by ‘let’ are automatically undone
when the ‘let’ is finished. The setting only affects expressions that
are inside the bounds of the ‘let’ expression. In computer science
jargon, we would say the binding of a symbol is visible only in
functions called in the ‘let’ form; in Emacs Lisp, scoping is dynamic,
not lexical.
‘let’ can create more than one variable at once. Also, ‘let’ gives
each variable it creates an initial value, either a value specified by
you, or ‘nil’. (In the jargon, this is binding the variable to the
value.) After ‘let’ has created and bound the variables, it executes
the code in the body of the ‘let’, and returns the value of the last
expression in the body, as the value of the whole ‘let’ expression.
(“Execute” is a jargon term that means to evaluate a list; it comes from
the use of the word meaning “to give practical effect to” (‘Oxford
English Dictionary’). Since you evaluate an expression to perform an
action, “execute” has evolved as a synonym to “evaluate”.)
Info Catalog
eintr: let
eintr: Parts of let Expression