"Rounding down" is an inexact term in mathematics. You generally won't find this phrase in a math textbook. Yet we all know what it means. If the number to the right of a decimal place is four or less (4,3,2,1,0), we may choose to discard all the numbers to its right. The number we select remains unchanged.
Rounding down is a means to approximate value, to simplify, to clean up our spreadsheets and make them clearer to read. It lets us arrange a number into the precise place we would like it to be.