what is rounding?

Rounding is a way of simplifying numbers. If the driveway of a house is 5 metres and 7 cm long we would usually just say it is 5 m long.
Saying it's 5 m long will be close enough most of the time.

Here is another example. The picture shows a stick of rock next to a ruler. The ruler has only got the 10 cm points marked on it.