The Basic Steps
1. Create a new composition with your video
2. Add a black solid on top of your video layer
3. Create a mask using the pen or rectangle tool wherever you want to highlight
4. Switch the mask type from Add to Subtract (or just click the inverted box)
5. Add an opacity transition to the black solid from 0-75% (or around 75%)
That’s basically it!
You can add feathering to the mask and add an animation to reverse the highlight as well.