A device is a phone, tablet or computer running the Mobile Data Anywhere app. Each device in your account has a Device Name and a Unit ID. The Unit ID is entered into the app and is how the device is recognised when it connects. All devices in your account connect with the same device password, shown on the device settings page.
You can add devices in two ways:
Mobile Data Anywhere is priced per license. A license covers one device, or one user with edit access.
The license allocation table on the Account page lists every license: which user and device each one is used by, and how many are free. Devices with no linked user show a "Link a user" link there.
Deleting a device stops it from connecting, but it does not remove a license from your subscription. To reduce your subscription, delete the device first, then choose "Remove licenses from my subscription" on the Account page. If the device was linked to a user with edit access, that user keeps using a license of their own until you link them to another device or make them read-only.