Diagon Alley was a protocol for selling stuff on the internet that never got implemented on LNbits.

One would create a shop (called a "stall") for selling stuff and accepting satoshi payments, but because it would be hard for buyers to find these stall there was this concept of an "indexer" which would get updates from the stall and display a nice webpage with all the listings from multiple stalls, then the user would choose the product and click and be redirected to the proper stall page.

Or something like that.