Slides for a brief presentation at a PhD Interview
1) Remember working on a PhD is a commitment and there will be times when you will want to give up so the question is "What will keep you going?" 

Motivation is a key factor in whether or not we stick to a task. This is the reason why they are asking you for this slide in particular. As an interviewer, I would like to know first if your motivation is intrinsic or extrinsic and then what specifically moves you to progress through a task or pursue this field.

2) Presenting in front of an audience when "something is at stake" can be nerve-racking. Your slides should be a way to quickly remind you what you need to discuss in case you block out. However, for the viewer the slides must be engaging and informative but not overwhelming. Even your thesis slides don't need to be overly technical because the interviewers are interested in whether you can present your findings to both technical and non-technical audiences, alike. I would recommend providing a high level overview first sprinkled with some technical terms but no details then as time permits elaborate increasing the technical level and vocabulary as you progress.