And the work of righteousness shall be peace; and the effect of righteousness quietness and assurance for ever.
(The Bible KJV – Isaiah 32: 17)
For to be carnally minded is death; but to be spiritually minded is life and peace.
(The Bible KJV – Romans 8: 6)
The calm and exalted thought or spiritual apprehension is at peace.
(Science and Health with Key to the Scriptures by Mary Baker Eddy p506: 10-11)
Christian Science Canberra 
that quote from Mary Baker Eddy so sums it up:
The calm and exalted thought or spiritual apprehension is at peace.
(Science and Health with Key to the Scriptures by Mary Baker Eddy p506: 10-11)
It is both the ‘spiritual apprehension’ – the gaining of spiritual understanding – and the application of that gain and understanding to everyday living that goes towards a calmer life (if that is what one seeks).