What is the difference between Initial timetoreply™ and Overall timetoreply™?

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