The "Chalk Talk," used primarily in the sciences and engineering, is basically a discussion of your research agenda without help from slides or formal visual presentation. In the talk, you're expected to communicate your research goals, the specific aims of your planned experiments, expected outcomes, and the reasoning behind your approach, including possible alternatives, should "Plan A" fail. As you do this, try to remain enthusiastic and engaging, rather than defensive.
If you want to avoid feeling like a deer in the headlight, practice, practice, practice. To join the FAS Postdoc Mock Talk Group in 2017-18, please contact either Myunghee Kim, email@example.com, or Barnabas Daru, firstname.lastname@example.org. The MockTalk schedule includes a list of ongoing talks in 2017-18.
Some additional resources:
Key point: You are not seeking the approval of your graduate committee or your advisor. You are interviewing to be their colleague and as such you need to show that you have opinions and ideas that are going to make you an important part of their department.