It's not a "mode" but simply a command that you launch on the command line.

If you work in 2D you wil get dimensions in the X and Y directions.

Launch the command DIMORDINATE and at the 2nd click indicate a place in your space, then you will be indicate ( allways take a look at what is written on the command line and eventually follow the instructions ) to choose an horizontal dimension ( to get an elevation from the point 0,0 ) or a vertical dimension ( to get a horizontal distance from the point 0,0 ).

Click on the dimension that you have placed and you will see that there is also a "grip" at the point 0,0, you can eventually grip a new the origin and so the different ordinate dimensions will be updated.

When you work with dimordinate you will have a big interrest in having your entire drawing "in situation" so that you can get good informations when you lauch the ID command or when you place ordinate dimensions.

Take a look at this :

http://knowledge.autodesk.com/support/autocad/learn-explore/caas/CloudHelp/cloudhelp/2015/ENU/AutoCAD-Core/files/GUID-A2513269-161C-4193-BBC4-1D6717C41A47-htm.html

http://docs.autodesk.com/ACD/2010/ENU/AutoCAD%202010%20User%20Documentation/index.html?url=WS1a9193826455f5ffa23ce210c4a30acaf-6188.htm,topicNumber=d0e132100