They exist in most European countries, and many others, as far as I am aware.
They are hectometer markers. Every 100m there’s one, often labeled like these:
Or like these:
The number in green is the location on the highway/freeway. Every one of these has a beginning and an end, one of these is labeled 0,0 and then every 100m all the way to the end of that highway there is a marker. In case of an accident or breakdown, it is very simple to inform the highway services where you are and get advise on that specific area.
The code in red is a highway number, the code in yellow is the code for roads with lower max speeds, both contain a max speed, if it isn’t very clear from the traffic situation, the box in yellow tells truckers if they could have height clearance issues, Li means links (left) and tells you on which side of the road you are and the lower sign also tells you which government institution is responsible for the road (in this case the province of Utrecht).
The top ones are also used to mark turns in the road. Their reflective red part is for that.