What is the difference between Initial time to reply and Overall time to reply?

Difference between avg. initial response time and avg. overall response time.
The initial time to reply (avg. initial time to reply) is how long it takes your agents to respond to a new conversation (first email in a conversation thread).

The overall response time (avg. overall time to reply) looks at the average time it takes to respond to all emails (initial reply and all subsequent replies).