What does Word mean by "passive voice"?

I have requested an agent to cancel my tickets in email as shown below

It helps if I get a confirmation once the request to cancellation of the ticket has been updated.

But, my Outlook (this is Microsoft email application, which uses the dictionaries and rules from Word) shows an error prompt for grammar correction.

It says "Passive voice (consider revising)"

So, what is wrong here? What is "passive voice"?