Visits is a metric.. for each visit or session for a user, the metric is incremented once.
So let's say I come to the site and look at 5 pages... In Adobe I will show as having 1 visit and 5 page views (which should be pretty standard, but I am using this as a base for how to describe the segment)...
The "All Visits" segment is essentially just a segment that should contain every single hit made in your suite. It's not specifically about the "visits" metric... rather this is an overarching segment that can represent the entirety of your traffic.
You should also notice that there is a segment called "Single Page Visits", this segment represents only the visits that had one page view during the visit.
Or "Visits from Search Engines" which represent all the traffic where the visit originated from a search engine.
Adobe is just providing everyone with some pre-defined traffic segments that that can use with standard metrics like Page Views, Visits or Unique Visitors (or also with most of your custom events).