The boost elbow is the brass elbow that is screwed into your compressor housing at the end of your wastegate actuator hose. The factory one is a clear pass through elbow, an adjustable one will have an allen head screw that you can adjust to squeeze off the flow, there will also be a weep hole, very tiny in the side of the elbow.
You really do not need one, but it will allow you to maintain higher boost levels without opening the wastegate, which I guess is most likely your goal.
Any of our aftermarket performance sponsors can probably help you obtain one.