つまり is a discourse marker that means 'in other words' or 'in short'. You use it when you want to restate something more directly, or when you draw a conclusion from what was just said. It often clarifies that the second part is essentially the same as the first, but expressed more plainly or with more focus. For example, if you explain a process in detail, you might follow with つまり to give the bottom line. It can also be used to summarize a longer explanation, similar to 'so basically' in casual English. It is common in both speech and writing, but in speech it often marks a deliberate clarification or a lead-in to a conclusion. When you use it, you are telling the listener: 'What I really mean is...' or 'Put simply...'. It is not used to introduce a new topic; it always links back to what was just said.