An approximate number is a number that is close but not exactly equal to another number.
Chad saw this sign at the grocery store: 3 candy bars for $1.00. At this rate, what was the cost of each candy bar?
$1.00 ÷ 3 is a little more than $0.33 and a little less than $0.34. Both $0.33 and $0.34 are approximate numbers for the cost of each bar.
In this case, Chad paid $0.34 for one candy bar.
See also about, approximate.