What is the meaning of [A charmed life]

A life of guaranteed good fortune or invulnerability, by virtue of a charm or spell.
From Shakespeare’s Macbeth, 1605.

Thou losest labour:
As easy mayst thou the intrenchant air
With thy keen sword impress as make me bleed:
Let fall thy blade on vulnerable crests;
I bear a charmed life, which must not yield,
To one of woman born.