|This item marks a mountain pass.|
|Used on these elements|
|Tools for this tag|
This item marks the highest point of a mountain pass way. Unfortunately, the term "mountain pass" is ambiguous (see Wikipedia). The meaning can be the whole road (or footway) up and down a mountain or the saddle point. Maps usually "special mark" only the latter (often with the corresponding elevation) and that's what mountain_pass=yes is meant to be for. If you want to mark the saddle point, see natural=saddle.
A mountain_pass=yes tag should be the highest point of the way so it's a single node on that way.
Applies to the "highest node" on a highway = motorway/secondary/footway/... (could be any appropriate "highway"):
Lots of passes are the border between two countries and therefore have names in (at least) two languages. These names should be tagged, e.g. name:it=Passo dello Stelvio and name:de=Stilfserjoch (but keep in addition a simple name=Passo dello Stelvio where possible, otherwise simple renderers may display no name at all). The corresponding language abbreviations can be found at ISO639-1 codes.
A lot of the well-known passes have dedicated Wikipedia pages.