Rounding approximates a number to a nearby ten or hundred. Rounding to the nearest ten helps estimate sums and differences. For example, 47 rounds to 50; 23 rounds to 20. Rounding develops number sense and checks the reasonableness of computed answers.
You've already worked with comparing and ordering three-digit numbers, so you understand where numbers live on a number line. Rounding uses that understanding to find the nearest "clean" landmark — the nearest ten or hundred — rather than the exact number.
Rounding works by asking: which ten (or hundred) is this number closest to? Picture a number line marked with multiples of ten: 10, 20, 30, 40, 50… If you have the number 47, it sits between 40 and 50. Is it closer to 40 or 50? Since 47 is 7 steps from 40 but only 3 steps from 50, it rounds up to 50. The simple rule that makes this automatic: if the ones digit is 5 or greater, round up; if it's 4 or less, round down. So 43 rounds to 40 and 47 rounds to 50.
Why does this matter? Because rounding lets you estimate quickly without doing exact arithmetic. If you need to add 47 + 23, you can first estimate: 50 + 20 = 70. Then when you compute the exact answer (70), you can check — does 70 feel reasonable given your estimate of 70? Yes! Estimation gives your brain a target, so computation errors become obvious. If you accidentally computed 47 + 23 = 90, your estimate of 70 tells you something went wrong.
The same idea extends to hundreds. The number 347 sits between 300 and 400; since the tens digit is 4, it rounds down to 300. This layered thinking — first round to the nearest ten, then check if you need to "re-round" to the nearest hundred — builds the number sense that will make mental math feel natural throughout school and everyday life.