Dot is a decimal point. What follows it is the representation of a decimal fraction, so it separates the whole number part of a number from the fractional part. In 5.8, 5 is the whole number and 8/10 is the fraction. 8/10 simplified to ⅘ as a fraction. Another example is 5.85. This simplifies to 5+85/100=5+17/20 so the whole number is 5 and the fraction is 17/20.