To keep the game simple and accessible to everyone, I source the solution words from a shortlist of 2,309 common words. Compare this to the full list of 14,869 valid 5-letter words in the dictionary!
A row will turn green every time it happens to spell out a word, even an obscure one. This is deliberately done so that you know that the Quintumble solution is unique, even when obscure words are possible. But just because a word is possible, it doesn't mean it will get you to the solution. Remember, the objective is to make 5 words in total. If you haven't heard of the word before (yet the tiles turned green), chances are this word won't feature in the solution.
Similarly, offensive or crude words will light up green, because they are all legitimate words in the English dictionary. But you can be sure that they will never feature in the solution.