In the expression f(x)=23, x is not a variable. f(x) means “a function of x” or “f of x” and is in practice another way of substituting for the variable y, that is, we are saying:
y=23.
That means x is unspecified and could be anything. So on the xy plane, this is a straight horizontal line, 23 units above the x axis. That would be the best way to interpret this, although it is a little ambiguous.
Usually functions of x are expressed in terms of x, such as:
f(x) = x^2
