What is the difference between the 'Agent' and 'Being replied' columns?
The [Agent] column indicates the agent that is responsible for handling a specific ticket. See the article Assign Tickets to Agents for more information.
The [Being replied] column displays the name of the agent who (at that point in time) is responding to an email message. The responding agent's name will be shown in this column for the duration it takes them to formulate and finally send their reply.