What is dwell time?

Dwell time is the duration between when a user clicks on a search engine result, and when the user returns from that result.

Dwell time is the total amount of time a user spends on a particular website

Actually Dwell time is the measurement of Google. Which tells us about UX(user experience).
Google notes that how much time a user spent on site. 

Dwell time is how long a user spends on a given page before returning to the SERPS. Dwell time, on the other hand, specifically refers to the time a user spends on a page before returning to the search results page.

Dwell time is the amount of time that passes between the moment a user clicks a search result and subsequently returns back to the SERPs.

Dwell time is the length of time a person spends looking at a webpage after they've clicked a link on a SERP page, but before clicking back to the SERP results.