When a measurement has been rounded, the true value is not exactly the number written. Bounds tell us the smallest and largest values the measurement could be.

The lower bound is the smallest possible value, and the upper bound is the boundary before the value would round up. For a length of 28 cm to the nearest cm, the interval is $27.5\le l<28.5$.

To find bounds, work out the rounding gap and halve it. Nearest cm gives a half-gap of $0.5$, nearest 5 m gives $2.5$, and correct to 2 decimal places gives $0.005$.