The average First Reply Time measures how long it takes your team(s) or team member(s) to respond to a new conversation (first email in a conversation thread).

The average Overall Reply Time looks at the average time it takes to respond to all emails (initial reply and all subsequent replies).

So while the First Reply Time is a measure of how quickly your team(s) respond to your customers for the first time, the Overall Reply Time measures how efficiently they handle the ongoing back and forth communication with the customer.