I have to guess what the context is because there is no more information, so if x∈ℝ (x is a real number), then I would guess that there is a function f(x) such that f(x)=(x-a)(x-b)(x-c)...(...)g(x) where a,b,c, etc∈ℝ and g(x) has only complex zeroes (and has degree 2, 4, 6, ... because complex zeroes come in pairs).
If f(x) is plotted in the x-y plane, then the curve only intersects the x-axis on the real zeroes. So the x-axis would be the number line.