my son is in 4th grade, it is a question for his math class

This question is talking about multiplying, for example, 123 x 3.

The most important step is to remember to multiply all three digits by the one digit.

So 3 x 3 = 9, 20 x 3 = 60, and 100 x 3 = 300, so 123 x 3 = 369.