Decimals are simply fractions written using a different strategy. Instead of writing denominators all the time. Decimals tell you the denominator by their place value. Tenths mean a denominator of 10. Hundredths mean a denominator of 100.
This interface is designed to show decimal multiplication, but first play with it to show decimals like 0.8, 1.8, and 0.8 + 1.0. Then, use it to see how each column is equal to tenths, but each square is equal to hundredths because 0.1 x 0.1 = 0.01. (You know this because a 10% tax on a dime is one penny.)