It basically means 9 things. However, please be sure to listen to the sound here because the number nine reads differently from きゅう or く. It is a Japanese native reading, and merged with つ. Numbers under ten are read in native readings before つ.
The examples in my dictionary are (since the course is elemental, I have translated them):
I cannot even see a star.
I have two kidneys.
There are three possible causes.
Divide the pie into four parts.
Enumerate five of the highest mountains in Japan.
How old are you? N.B. This sentence using つ is for asking children's ages (who is evidently under ten). When said to grown-ups, it seems impolite and even insulting.
The rule is when you are not sure about 'counters', use this general つ, but this is usually used when the quantity is under ten. The Japanese counters are very hard and sometimes even confuse native speakers (as the case in other East Asian languages -- Chinese, Korean, and Vietnamese). There are even 'counter' trivia in TV quiz shows.
九つ used to mean also noon or midnight. It was the Japanese clock (https://en.wikipedia.org/wiki/Japanese_clock).
That's the essence of furigana. I know a lot of people want it here, but if they do use it, we will rely on it more than learning kanji. Clearly by including kanji in our lessons, Duolingo believes it is critical in learning Japanese, and, frankly, I would tend to agree with them
It's a lot to take in all at once. A lot of things in Japanese have multiple readings, etc., many of which can be attributed to the Chinese writing system influencing the entire language, so it can help to focus only on one half at a time. Duolingo also often splits up a category across multiple lessons so you don't get flooded with information. When I ran through the German module, they only do 1-10 to start, then later cover the teens, and then 20-100 in still another later lesson.
There's even far more complexity to the Japanese numbers that hasn't been covered yet: tons of nouns have specific counters to keep track of. Imagine how we have to say a "pair of" for individual pants or scissors, but doing that for everything, all with different arbitrary counters.