It means Actual time spent on a webpage before returning to SERPs page.
(For example – user searches for an article and then he clicks on first article then he read and stay for 15minutes. And he return to back search page (SERPs).
The time he spent on that page that is Dwell Time i.e 15 minutes in this case.
There are two type of bounce Time
1) Standard Bounce Time –
If user clicks on an ad and he goes to the landing page and finds irrelevant content/high page load time/ then, if he goes back with in 6sec i.e Standard Bounce rate.
2) Actual Bounce Time – If user searches a query, clicks on an ad and then he stays on the webpage but he did not click anywhere in the page and go back to SERPs, then that is the actual bounce rate.
On Page Time
The time spent on a webpage before going to another webpage within the website.