Relation between Traffic group and device group

You can say anything you like. Whether people will understand is another thing. Technically a traffic group is a collection of related traffic objects that reside in a sync-failover group on a BigIP Application Delivery Controller.

How much more or less you need to add to the explanation depends heavily on your audience. A group of F5 engineers wouldn't need any more than "Traffic group." A Cisco engineer who has never used a BigIP might benefit from something more complete like:

A Traffic group is a collection of related IP addresses that move between F5 BigIP Application Delivery Controllers in a high availability failover event.