[sentence]。つまり、[sentence]。 [noun]、つまり [noun]
つまり means "that is", "in other words": what comes after it says the same thing as what came before, more plainly or more exactly. It opens the second sentence, or sits between a word and the plainer word that explains it.
After a fact, つまり gives what follows from it: "which means". The sentence often ends in ということだ.
As a question, it checks that you have understood the other person.
Both can cut a long explanation down to its point. ようするに要するに does only that, and it can sound impatient, as if the speaker wants the talking to stop. つまり is neutral, and it also restates something short, which 要するに doesn't do.