Thousand
numeral
The kanji and the word are exactly the same. That means they share meanings as well.
せん
Normally a single kanji making up a word like this means it will use the kun'yomi readings. Numbers are generally the exception, though, which means this word uses the on'yomi reading. If you learned the kanji, then you should already know this!
一リットルとは、千ミリリットルのことだ。
One liter is a thousand milliliters.
そのフランス人は、千に一人の天才です。
That French person is a one-in-a-thousand genius.
千日くらいかかりますね。
It takes about 1,000 days.