Phase

Phase is the sign of an amplitude, or its angle when the number is complex and therefore has a real part and an imaginary part. A measurement in the usual 0-and-1 arrangement does not print the phase, so a gate has to turn that angle into a difference of chances before a click can show it.

Keith treated the sign as decoration for most of a week, a flourish you could leave off the diagram. The week ended when a Z gate on the |+⟩ state, followed by a Hadamard, turned a hidden minus into a certain reading of 1.