The "keys" under the banner tab refers to "categoryDesc" in the product data.
Basically the “key” links the banners imageURL to the correct banner.
Collection > Display > Banners >
As an example I am using my own demo collection.
Within my product data I gave the product a categoryDesc.(To categorize products in the app)
In the backend you can link the banner to a category (with the setting: Keys) (in my example: Pants)
This “links” the "imageURL" to the correct banner within a category.
No Key (not filled in) = Front page banner (category overview)
* = One image for banners without a key (You can use this for one generic banner)
If you want a banner to be generic (no specified category) use the wildcard *. This will apply the banner to every location that has not been specified with a “ banner key”