Time on page is a measure of the length of time a user stays on an individual page. It's usually presented as an average of the total length of time that all visitors spend on a page.
If you have a lengthy blog post and see that users are only spending a couple of seconds on the page before moving on then it's safe to assume that the page is not very useful. It's in your interest to improve the content here to create a better user experience.
Some sites like Medium, in an attempt to encourage users to remain on the page and read the entirety of the content, state the average length of time the content takes to read at the top of the page.