This page explains it pretty well.
As a numeric the allowable range that can be stored in that field is -10^38 +1 to 10^38 - 1.
The first number in parentheses is the total number of digits that will be stored. Counting both sides of the decimal. In this case 18. So you could have a number with 18 digits before the decimal 18 digits after the decimal or some combination in between.
The second number in parentheses is the total number of digits to be stored after the decimal. Since in this case the number is 0 that basically means only integers can be stored in this field.
So the range that can be stored in this particular field is -(10^18 - 1) to (10^18 - 1)
Or -999999999999999999 to 999999999999999999 Integers only