Where we turn a decimal number into a sum of each digit multiplied by its Place Value (tens, units, hundredths, etc):
Example: 37.2 = 3x10 + 7x1 + 2x(1/10)
Sometimes the expansion goes forever:
Example: 1/3 = 0.33... = 3x(1/10) + 3x(1/100) + 3x(1/1000) + ...