要するに means "in short" or "to sum up." You use it when you want to boil down a long explanation, a complex situation, or a series of events into one essential point. It signals that what follows is the core takeaway, often cutting through details or excuses. It is common in both speech and writing, though in casual conversation you might hear it slightly less than in formal settings; it is not stiff or literary, but it does carry a logical, explanatory tone. When used at the beginning of a sentence, it usually summarizes the previous statement or conversation. It can also be used mid-sentence, but the most natural placement is at the start. In spoken Japanese, you might hear it followed by a pause, like "要するに、…". The pattern can also appear at the end of a sentence as 要するにだ (less common) or with です in polite speech: 要するに、そういうことです.