It is a complex function of a real function of a variable.
It is created via icon of the numerical objects expandable bar (third form the bottom).
A real function can by modified with icon of the top toolbar.
For instance, if you want to define a complex function f with f(z) = (z + 1/z)^2, you can choose t as formal variable and enter formula (t + 1/t)^2.
See also : Complex calculus syntax.