Literally, a raft. There were always those places where someone would build himself a vikendica on water, possibly by connecting a dozen barrels into a platform and then building a little cabin on it, then tie it somewhere to a river bank.
Later, these became somewhat regulated, and also got the power and water, possibly phone and even gas. During the eighties, the old habit of mooring defunct ships and using them as restaurants on water has extended to refurbishing these rafts into restaurants, then into building new restaurants as rafts.
These perhaps exist in other places, but something above 90% of them are on Danube and Sava, in Belgrade. For the last two decades, at least, they are the focus of night life (including occasional swimmers, who get too drunk and fall over the fence). Many of these places are just fancy, with weird sounding items in their menu (I once tried black rice - black with squid ink - a bit expensive and nothing to write home about). They seem to be the interesting place for tourists, as the ambience truly is unique - there's a splav after splav for a couple of kilometers, and on Sava you can see your counterparts on the opposite bank. Being on the river bank also means they are off the main thoroughfares, so you get that feeling of not being quite in the city, while being only 200m from the nearest bus stop.