What are the most concrete and easiest to understand applications of deep learning in the industry?
This seems like a matter of opinion to me, but here are some applications of machine learning that you seen very often in Silicon Valley:


Advertising: what to show whom and when? The data comes fast and furious so the engineering part is quite sophisticated.
Recommendation: similar to the advertising problem, except there are no advertisers, and the volume is slower, so you can afford to use more sophisticated machine algorithms.
Security: did somebody outside penetrate your system? did somebody inside abuse your system?
Text/audio/video processing: information retrieval on media. Requires finding good representations of the items using machine learning; e.g., fetch all pictures similar to the input, where the similarity can be construed in various ways, such as similarity of texture, style, color, content, etc.


These are not necessarily simple, but they're representative. If you want to give a demo, I suggest the last one. You could show them how to find similar images.