URL contains a number after node/nid: why? I have just inherited a site to maintain.

The site has many pages ending with /node/$nid. I notice that appending /x to the end of this URL seems to change nothing. That is, if I browse to http://example.com/node/123/1, I get the same result as if I browse to http://example.com/node/123/2.

What are those numbers typically used for? I notice that in my sidebar, it seems to be sending people to the /2 version.
Drupal's path-matching is greedy. The path node/%nid will match anything with that string at the start, that has a separator after it. e.g


node/%nid/1
node/%nid/giraffe
node/%nid/barack-obama
node/%nid/this/is/just/too/much


The 'extra' path segments are passed on as extra parameters to the item's page callback, whether it wants/uses them or not.

The page that you see at node/%nid/1 isn't different at all from the standard node/%nid page. 

You'd do well to keep those URLs out of the page source at all costs, or make sure you have an appropriate canonical URL set, as Google will consider them duplicate content otherwise.