This refers to tablet personal computers. These are mobile computing devices characterised by touchscreen interfaces. These have been popularised by he introduction of the iPad and its competitors.
These devices present particular challenges for UI developers. Functionally they can be seen as sitting between high-end mobile devices and desktop computers. Like mobile devices, they are portable, so they give opportunities for geolocation functionality and the UI interaction have more in common with their smaller counterparts. On the other hand, like desktop devices they have a larger screen area which needs to be acommodated to during development.
The touchscreen UI mean that client-side UI interaction on the web can fail. For instance, click events are not valid. Frameworks such as jQuery Mobile give a larger class of events that can be leveraged by these devices, for example, swipe.