Intuitive

CPT symmetry means that a mirror-image of our universe where we reverse all momenta (corresponding to the reversal of time) and with all matter replaced by anti-matter would evolve under exactly the same physical laws.

Concrete

• See WHY IS CPT FUNDAMENTAL? by O.W. Greenberg

Abstract

• The standard reference is "PCT, spin and statistics, and all that" by R. F. Streater and A. S. Wightman.

Why is it interesting?

According to the famous CPT theorem the symmetry CPT holds for all physical phenomena under quite reasonable assumptions.