It is a crossroad where all the roads have a "stop" sign.
In Belgium/France (and Spain I guess) you usually have a main street which has priority on the other streets, or the cars coming from the right have priority.
But with the 4-way stops none of the streets have priority: it is the car coming first which passes first. I still wonder how that actually works (I think it would be impossible in our countries), but I guess it works if they use that