In the world of computers, the term “Pinned” is a lot like the physical act of pinning something up on a cork board. In our case, it refers to items within a window that are kept in the same place for easy access. Programs, applications, web sites, etc. can all be stuck up in a menu and accessed at your leisure depending on the program.
The bottom section of the Start Menu is reserved for frequently used programs, or programs that I use on a regular basis. Though this may sound the same as a pinned menu, the items on the list change dynamically based on what I've been apt to do lately. So if I've been using the calculator a lot, it's icon will show up, but if I stop using it for awhile and make use of something else, that icon will show up.
Get the point? :)
~Andrew