Subfile is a group of records in a record format and can be read from or written to the display in a single operation.
This sentence will make sense if you are an
AS400, iSeries IBM i Software developer.
If you are not then it probably sounds like gobbledy gook.
So what it means is - when a program builds a list of information and then wants to display it on the screen its called a subfile. Think of it as a collected set of information that is going to be displayed to the user. The user can generally scroll up and down through this list of information.
Example - a program wants to display a list of all the books you have purchased from Amazon. The information could be collected and formatted into a subfile and then the subfile would be displayed to the user