Vowel harmony is a feature in some languages (e.g. Turkish and Finnish) whereby vowels are divided into two or more groups, and each word in the language will only have vowels from one group. For example, a language could divide vowels into one group of a, e and i and another group consisting of of o and u. In this system, words like beliba, and otuto are possible, but aruli is not (as u cannot appear with a or i).
Vowel harmony often groups vowels based on some kind of feature, e.g.: high vs. low vowels, front vs. back vowels, rounded vs. unrounded vowels, or nasalised vs. unnasalised.
Languages may have three groups. In such cases it is often that the third group is a set of “neutral” vowels that can be appear in either of the other two groups.
In Vulgarlang, you are able to create two groups of vowels, and each word will be generated using vowels from one or the other groups. If you want to simulate neutral vowels, you can put some vowels in both groups.