Sample Problem 2 explains exactly what “rounding off” means.
Learning to round off is a valuable skill at this age when children need to be learning how to manipulate as many numbers as possible “in their heads” without resorting to pencil and paper. You will probably discover that while children can round off on paper, larger numbers, or add together numbers by rounding them off to single digits followed by however many 0s are needed to permit that, they may fumble completely if asked to do this “in their heads.” So, start simply. If rounding off on paper is a challenge, start by using this tip: It is often helpful for children to underline the number that they are rounding to. For example, if children are rounding to the nearest ten, underline the number in the tens place. Follow Learning Tip 2 for another assist to using mental math more confidently. Above all, progress slowly, using small numbers as long as necessary. Becoming confident at manipulating numbers without needing pencil/paper is a very valuable skill and is worth spending the necessary time to accomplish it while children are still young.