The default value is 0.5metres, but as seen in a number of topics, and in my own experience, this chops the tops off objects that you might want to retain in DTMs.
Here is a 3D model of a gravel stockpile beside the road. The piles unloaded from trucks are around 1metre high, the highest being a bit over 1.3m.
And here it is in DTMs with smrf_threshold: 0.1 (metres), stockpiles and vegetation removed quite close to ground level.
smrf_threshold: 0.5, much more of the gravel is shown
smrf_threshold: 1.5, the whole stockpiles are visible
The Digital Surface Model- essentially identical to the DTM with smrf_threshold: 1.5