In mathematics, a rational number is one that can be expressed as a fraction of two integers--for example, 1.4 (7/5), 0.333... (1/3), 2.85714286 (20/7), all of the whole numbers (e.g. 4=4/1), and so on.
An irrational number is one that cannot be expressed as a fraction of two integers. Some of the most famous irrational numbers are π, e, √2, and φ.
In other words, rational numbers can be expressed simply and exactly, but irrational numbers cannot. They are irreducibly complex.
Some of the most useful, important, and beautiful numbers in mathematics are irrational. But if you want to make use of them, you must strip them of context, and when you do, you're using an approximation at best.
When faced, as we are, with the overwhelming rationalism of bureaucratic capitalism, which seeks from its centralized perch to dictate to everyone else what makes sense in different localities, which seeks to define what is rational and to define everything else out of existence, I think it can help to remember these definitions.