That’s the indicator for a feedback loop. It shows you where the feedback delay is present. z is a variable for delay, so z-1 is a single sample delay, for example.
It’s not a letter that is put there with a node like these others were, it just automatically appears. If you use the FeedbackDelay node or the UnitDelay node, you can explicitly determine where the delay occurs. If not, Audulus will just try to guess where to put it.
You need a feedback delay because you can’t compute a result that depends on itself instantaneously. Like create an expression node where 1+x=x and attach a value node - you’ll notice it count up really quickly - that’s because there’s a small delay in the calculation of the input x and the output of the 1+x expression. Use a UnitDelay between the output and x and it will count even faster.