This will be a test of the launch escape system which will pull the capsule away from the rocket in the event of a booster malfunction during launch. This is the last major hurdle that SpaceX must overcome for Falcon 9 / Crew Dragon to be man rated. If all goes well, they will be launching astronauts from US soil in a few months.

SpaceX will simulate this by shutting down thrust on the booster right around MaxQ - or when the vehicle is undergoing maximum aerodynamic stress. The capsule will detect the shutdown and the SuperDraco rocket thrusters will fire, pulling the capsule away from the rocket.

I expect the booster itself will be torn apart by aerodynamic forces, so that could be a pretty exciting part of the text. For those interested, here's an animation of the planned test. Note that while the video is only 56 seconds, the full flight time will be closer to 10 minutes.

https://www.youtube.com/watch?v=qObBRM4euxk