When you enable the URL segments option for a given template (Templates > Edit Template > URLs > URL Segments) then PW will accept non-existant URLs, so long as they start with one that does exist. So lets say that you have a page called /products/widget/. If URL segments are enabled, then you could access any URL below that and it would be sent to /products/widget/. So accessing /products/widget/photos/ (a page that doesn't exist) would still get sent to /products/widget/. (If URL segments are not enabled, then of course accessing /products/widget/photos/ would generate a 404 instead.)
Using the /products/widget/photos/ URL, the 'photos' portion would be considered URL segment 1. You can check the value of URL segment 1 in your template file with $input->urlSegment1. If there were another segment on there too, like, /products/widget/photos/cats/ then 'cats' would be URL segment 2 ($input->urlSegment2), and so on.
The advantage of URL segments is that you can have just 1 page handling many URLs. The portion of those URLs that are translated to URL segments are not pages in PW. In fact, if you actually created a page in PW named /products/widget/photos/, then your 'widget' page would no longer receive the 'photos' URL segment, as the new photos page would then have control over it.