so for a example 3.5'' meins 3 and one hafe in inchs's.
Clearly you understand what the number 123 means.
It means one lot of one hundred added to two lots of ten, added to three lots of 1. I still remember doing this when I was at school a looooong time ago
The same thing keeps happening for larger numbers, so there is the position for thousands, ten-thousands, hundred-thousands, and millions, etc., etc..
Well, it also goes the other way, for things smaller than 1. And the decimal point just shows us that the stuff to the left of it are whole numbers. So wheat is to the right are fractional parts.
so 4.125 is four lots of one, one lots of a tenth, two lots of a hundredth, and five lots of a thousandth.
As a fraction, that is the same as as 4 + 1/10 + 2/100 + 5/1000, or 4 plus 125/1000.
With a bit of division by common factors, this comes to 4 1/8 -- four and one eighth.
As CocaCola says, this is true of ALL math -- it's nothing special.