This page is all about using mental strategies to add decimal fractions. Before attempting this children should be able to add two 2-digit whole numbers in their heads and have a good understanding of place value.
The mental process of adding 2.4 and 3.7 is almost the same as adding 24 and 37; in fact many people just ignore the decimal to work the answer out and then insert it at the end. Again, it is perfectly normal to add the units first and then the tenths, which is the opposite of doing it on paper.
The second half of the page looks at adding tenths and hundredths. A good way to look at this is to think of it as money ie adding pence and writing the answer as pounds (but without the sign!)