Feeds are the core of the Adafruit IO system. The feed holds data (say, the temperature) and metadata (such as the timestamp) about values that get pushed to Adafruit IO. You will have one feed for each type of data you send to the system. For example, if you have a project with temperature and humidity sensors, you will need to create one feed for temperature and one for humidity.
Feeds also contain overall information such as descriptions about the data, whether the data is public or private, and when the last value was pushed to Adafruit IO.