The model deals with database interaction and will (should) contains functions that modify records in database. Although CodeIgniter doesn't force you to use this layer, but generally it is best practice to use it.
The view acts as display system. Giving the browser something to display. It could be html pages, xml or text files.
The controller works as the middleman between view and model, it is a place to load your library, plugin, call your model functions and sent data to the view.