Feeds is any form of external data that you want to periodically update in your Repositories.
Say you have a catalog feed or pricing feed or any other type of custom feed.
You usually get feed in the form of file in various formats like xml feed file, xl or csv feed file.
The main purpose of the feed is to extract all the data from the feed file and create/update that data in your repository in atg, so that your application can use that data.
Feeds usually are executed via schedulers or webservices, so it can periodically update the data in your repositories.
You can write a scheduler or a webservice program with a relevant parser logic that parses the input feed file and extracts each record data and puts it in your repository.