Devices are the Android phones/tablets that you can use to collect responses once you have created a form. These are generally synonymous with your surveyors or users. Once a form is published, access to the form can be provided to multiple devices which can then use the Collect Mobile app to collect survey responses.
A device on Collect is said to be active if at least one response has been submitted using the device during the subscription period.
Here's how you can add devices or members on collect.
Not sure if a device is compatible with Collect? Here's a list of some of the devices which support the Collect app. You can also read about basic device specifications here.