The adverb therefore means as a result or consequence, or "for that reason." Cats and dogs tend to fight; therefore, keep your pets apart at night if you want to get some sleep.
The adverb therefore is also used to introduce a conclusion that follows based on logic. You'll see this construction a lot in logic, math, and science, for example. Of course, sometimes the conclusions are false, as in this argument: "All fish live in water. A whale lives in water. Therefore, a whale is a fish." A whale is, in fact, a mammal. Therefore, the argument that a whale is a fish is wrong.