When rounding, if a number is greater than or equal to 5, you round up. If it is 4 or less, the number place you're rounding remains unchanged.
Since an 8 is in the tens place, you round up.
382 is 400 when rounded to the nearest hundred.
You can also think of it as which hundred it's closer to.
There's 200, 300, and 400.
Which of those is 382 closest to?
400 - 382 = 18
382 - 300 = 82
382 - 200 = 182
18 is the lowest, so there is the shortest distance between 382 and 400, so 400 is the closest hundred.