Rounding to the nearest ten replaces a number with the closest multiple of 10. Numbers ending in 0–4 round down; 5–9 round up. A number line visually shows which ten is nearest: 34 is closer to 30, but 37 is closer to 40.
You already understand place value — that a number like 47 is made of 4 tens and 7 ones. Rounding uses that structure to replace a precise number with a nearby "round" number that is easier to work with. When you round to the nearest ten, you are asking: "Is this number closer to the ten below it or the ten above it?"
Picture a number line with the multiples of 10 marked: 30, 40, 50, and so on. Every other number lives between two of these markers. The number 34 sits between 30 and 40. Is it closer to 30 or 40? It is 4 steps away from 30 and 6 steps away from 40 — so it rounds down to 30. The number 37 is 7 steps from 30 and only 3 steps from 40 — so it rounds up to 40. The ones digit tells you which ten is closer: ones digits 0–4 mean round down (stay at the lower ten), ones digits 5–9 mean round up (go to the higher ten).
The rule for 5 is a convention worth knowing: when the ones digit is exactly 5, the number sits perfectly halfway between two tens. Mathematicians chose to round up in this case — it is a decision, not a mathematical necessity. So 35 rounds to 40, and 45 rounds to 50.
To round any number in your head: look only at the ones digit, ignore everything else. If the ones digit is 0, 1, 2, 3, or 4, replace it with 0 (round down). If it is 5, 6, 7, 8, or 9, add 1 to the tens digit and replace the ones with 0 (round up). So 73 → 70, and 78 → 80. This skill is the direct foundation for estimating multiplication and division, which you will use immediately in the next topic.