The term "input" refers to both the:
Physical connection of a cable from one home entertainment device to another
Default name for such a connection in a device's software menu
For many home entertainment devices, the physical input ports (usually found on the back of the device) are directly linked to their default names. For example, if you have a game console connected by an HDMI cable at the rear of a TV to input "HDMI 3", you'll likely be able to use it by setting the TV to "HDMI 3" in the TV input menu.
Before you try to configure your Harmony remote activities, you'll need to know which device:
Is used for each activity
See answer 360023250854 for more information on Harmony activities.