The "parity" operation is thre reflection of all thrre axes,
x-->-x, etc. Until 1956, the assumption was made that changes the axes in this way did not affect the result of any experiment. That was called "parity conservation".
Conservation of parity is violated in weak interactions because the basic weak interaction is a linear combination of a vector (V)and an
axial vector (A). (The EM interraction is pure vector and conserves parity.) The violation is said to be "maximal" because the effective interaction term is V-A, so that the V and A have equal effect, rather than having just a little A.