Named or per-seat license can be used only by the BPMonline user assigned to it. Even if that user is not logged in at the time, their license can’t be used by others. The up-side to this option is that the named user will always be able to log into the software regardless of how many other users may be logged in.
Concurrent licensing is similar to ports on a phone system. Only active logged in connections count. With Concurrent licensing, your organization can create as many accounts as you like. They only limit is the number actually logged in and using the service.