Device ID is a unique ID that GoCanvas assigns to each device that logs in to GoCanvas. You can find the Device ID for your users by going to Account>User and clicking on the name of the user.
If you want to confirm what device a Submission was completed on then you can export the Device ID along with your Submission data. Go to Submissions and click on the Export button next to the form you want the data for. From the export screen you can check the box to include the Device IDs.