What is the meaning of Let_alone?

Much less; to say nothing of; used after one negative clause to introduce another, usually broader and more important clause, whose negation is implied by the negation of the first. However either of these instances mentioned can be applied with the use of let alone.

Not to mention, as well as; used after one item, to introduce a further item which is entailed by the first.

To leave alone, let be; to stop bothering.

To rely on (a person) to do something.

Source: wiktionary.org