In short: It means "Don't assume any of the usual meanings of a broken line in Spain, except for marking the center of the road".
The meaning is obvious. At least, it seems obvious for drivers in Spain.
In Spain, a broken line means that you can overtake while a continuous line means you can't, and in curvy roads the line changes very often from broken to continuous (and even to continuous for one side and broken for the other) according to circumstances of the road - mostly about visibility but also about intersections with other roads.
In roads with that sign, as it reads, the broken line just marks the center of the road and it doesn't show whether you can overtake. Therefore, you must assess if you can overtake using the other rules in road code, which basically fall back to the rule that you can overtake where visibility is enough to make sure that there isn't an incoming vehicle in the opposite direction.
If the road were narrower than the one depicted in the OP, the sign would be also warning that we can't assume that every vehicle will be completely in its side of the line, especially for trucks. In that case, there would likely be a speed limitation signal.
Addition to address comments concerning why aren't the whole road (or the unsafe parts) painted with continuous line:
Those roads are just small roads with little traffic, some of them just upgraded from unpaved tracks. That kind of small roads in some places in Spain used to carry no lines at all. A better improvement could include complete road marks and a lot of traffic signs, but that improvement is nor easy not cheap, and for small roads with little traffic it is not done. Therefore, the road is just an asphalt strip with a broken line in the middle.
From experience, I must add that it isn't hard to drive in those roads - at least, lack of continuous/broken line doesn't make it harder than driving in any other narrow curvy road. As any driver knows, you can't overtake if you can't check that there is no incoming vehicle, and you don't need a continuous line to tell that you can see the road more than a few metres ahead because a curve obstructs the view.