The Rationale slide identifies needs for design. Have a look at the examples below.
A Rationale is a justification as to why you are doing the project you have chosen. Think about it like: why did you choose the drinks bottle you have? Was it because of the colour? The materials? What it looks like? How much it costs? etc.? Now apply that thought process to your chosen project idea. Why are you doing your project? Why is there a need for it? Who wants it? What do you hope to learn from it? Why is it important to you? Is there any research that backs up why it is a project that needs to be developed?
The rationale explains the initial idea in much more detail, justifying it with any research or data and explaining why it is important.
Note that Rationale is incorrectly written as 'Rational'.