Explanation.

Talked the other day about irrational numbers (0.999... remember?).  This is one of those words which have a "technical" meaning which is not the same as its everyday meaning.  In normal speech, irrational is used as a synonym for illogical or unreasonable, but in mathematics it has a specialised meaning based on the root word "ratio", which is a rather old-fashioned term for what today we would more likely call a fraction.  So an irrational number is simply one which cannot be expressed as a ratio, a fraction.  So any number which you can't write in the form of a/b (where a and b are integers - whole numbers) is an irrational number.