In monetary terms, you're making change. If you need more ones, you trade one 10 for 10 ones.
When I was in school, many years ago, we used small superscript-type numbers to show this:
7 3 0 ⇒ 7 2 ¹0 . . . . regrouping to gain 10 ones
7 2 ¹0 ⇒ 6 ¹2 ¹0 . . .regrouping again to gain 10 more tens
You'd have to look at the example problems that precede this page in order to see what the meaning of "magnifying glass" is.
