What is the difference between first reply time and requester wait time metrics?

Question

What is the difference between first reply time and requester wait time metrics?

Answer 

First reply time is the time between the ticket creation and the first public agent comment on the ticket.

Requester wait time is the time a ticket spends in the New, Open, and On-hold statuses. If you have custom ticket statuses enabled requester wait time will reflect the time spent in New, Open, and On-hold status categories. 

For more information about SLA metrics, see the article: Defining and using SLA policies.