This may sound like an elementary question, but you would be surprised how many speakers try to prepare a talk before clearly defining their subject.Often, a presenter can give you a general idea of their topic, but not a specific one; it is this absence of a clearly defined subject that is the root cause of many subsequent problems.
Here are just two.
First, a fuzzy subject makes doing research difficult and, therefore, much more time consuming.
Second, it creates confusion when you are trying to determine the goal of your presentation.
Developing a clearly defined subject is so important that Chapter 3 is devoted entirely to this topic. For now, suffice it to say that the sooner you know the subject of your talk, the better.
Once you decide on a subject, make sure you can say it in simple terms. Speakers should be able to articulate clearly the reason for their talk in just a few words. Below are some pithy titles that leave little need for explanation. They pack a punch and make the subject matter clear to the casual observer: