Pinyin: the "ü" vowel
So you've learned 5 vowel sounds already, but those are not all the vowel sounds in Mandarin Chinese! The “ü” vowel is a totally different sound, and it does not exist in English. If you speak French or German you might be familiar with this sound, but otherwise, you probably have to train your mouth to make a whole new sound.
Also, don't think that “ü” must be basically the same as "u." They're not the same sounds; those two dots make a big difference!