XBMC's UPnP client (aka UPnP renderer) allows Kodi to instantly playback and control videos and music sent to it from a UPnP server and control point. This is similar to the AirPlay protocol, but more flexible and open. Several applications exist for nearly all major OSes that can send Kodi media through UPnP. This is different from using UPnP as a media source, where Kodi browses a UPnP server, rather than being controlled by one.
UPnP client settings
To turn on XBMC's UPnP client, in Kodi go toand enable "Allow control of Kodi via UPnP".
Sending video to other UPnP targets
As of v13 "Gotham", Kodi can also be a control point in addition to being a UPnP server, allowing you to "beam" the video to another device.
Then when you want to play something, bring up the context menu while that video is selected, and choose "play with". That should give you a list, starting with DVDPlayer (local), and any UPnP renderers that Kodi is able to detect.
If playing to another instance of Kodi, remember to turn on the UPnP setting for allowing UPnP control:.
Kodi still needs some tweaking in the UI to make controlling a little nicer, but that can also depend on various skinning options. For example, on Confluence you have to use the sidebar menu's "mini player" controls, but on a tablet using Touched or Retouched, you can use the full screen controls or the larger "mini player" controls at the top.