"Under" a number means "less than" that number. But "under" is used in certain specific situations:
You use "under" to show that something is below a limit:
They don't allow anyone under 18 to enter.
I was driving 10 miles under the speed limit.
You use "under" a number when you're emphasizing that the number you're comparing to is already low, like in the example at top. Some other examples:
We have DVDs on sale for under ten dollars.
They promise to deliver it in under 30 minutes.
The opposite of "under" is "over". The opposite of "less than" is "more than".