Something that's conjunctive tends to connect or combine two things. A conjunctive effort between two people is a combined attempt to get something done, and the word "and" in this very sentence is conjunctive since it's joining two phrases together.
The word conjunctive often comes up in medical terminology, in phrases like "conjunctive tissue," or tissue that connects different parts of the body. Another kind of conjunctive is a part of speech — like the words "but" and "or" — that connects phrases or words within a sentence. Conjunctive comes from the Latin word coniunctivus, "serving to connect," with its root of coniungere, "to join together."