There is a traffic sign used in France meaning "end of build up." But what does that mean?

End of heavy traffic zone, perhaps?

Build up area, more developed area, city and developed roadside business area with more traffic lights, leaving for less built-up, more rural areas.
search “left the build-up area” on wiki.openstreetmap

@anartist Has it correct.

Interesting – French road signs are almost identical to UK ones, although we don’t have an “end of build up” sign we have one that means a similar thing – end of speed restriction which basically means that national speed limits apply instead of local ones. It nearly always occurs as you leave a built-up area.

In the Netherlands we would refer to it as leaving the residential zone of a municipality. On the other side would be a sign welcoming you to – for instance – the municipality of Amsterdam or residential zone of Hoogland. All in all similar to @anartist‘s explanation.