What is the meaning of "Amplitude of Detection" in A. Zee's QFT book?

[Golak Bage]

TL;DR Summary

Zee explains "The amplitude for detection is given by a fundamental postulate
of quantum mechanics, the superposition principle, as the sum of the amplitude for the
particle to propagate from the source S through the hole A1 and then onward to the point
O and the amplitude for the particle to propagate from the source S through the hole A2
and then onward to the point O." [Page. 3]
I want to understand what does he considers "the states" in the slit experiment context ?

 

[Demystifier]

Zee takes the philosophy, common among many quantum field theorists, that path integral quantisation is sufficient, so that canonical quantization is not really needed. The problem with such philosophy is that the notion of state remains vague. My advice: learn quantum mechanics first, from a book which is not primarily about quantum field theory.