Many peoplee take that to mean flux, such as that everything is constantly changing, but that interpretation doesn't work with anatta. The interpretation of "Instability" does though, for example you have no say over the matter (anatta) if all of a sudden a headache or joint pain arises (anicca). Things are inherently unstable (anicca) and thus out of your control (anatta).
Here's a good essay that expands on the different interpretations
The sun courses daily across the sky; the seasons progress annually; and this in itself does not induce anxiety. Rather, I should be disconcerted and grieved if the sun were to stop transiting the sky, or if it were to remain always winter, or even always summer. This would be truly upsetting. Yet this is not so much a matter of change as of becoming otherwise, i.e. other than the way I want or expect things to be. The sun’s position has stopped changing “all the time;” the seasons have ceased their advancement. This is the sort of change I turn from and wish to deny. For even if matters were not arranged in their most perfect possible order they were at least arranged: day followed night, winter followed autumn. There was not the threatening anxiety of uncertainty: if this, what next?
But the doctrine of flux is a doctrine of certainty: everything is always changing. It is therefore a falsification of our manifest awareness of the world’s unreliability: things change when we expect (and wish) them not to. The need to hold to and proclaim this doctrine is thus revealed for what it is: not a coming to truth but a fleeing from it. In the face of the world’s insecurity the doctrine of flux is an attempt to retreat into a position of certainty.
https://pathpress.wordpress.com/bodhesako/change/Rather than perceive impermanence as the decay and decrepitude of old age, as the weakening of the faculties, the loss of control over the body, the gasping for air as life ebbs, the fearsome uncontrollable slide from light to darkness as our very identity — body, perception, consciousness, all — fades away and breaks up — rather than perceive impermanence as that, how much more comfortable to blandly assert that everything is always changing, and thereby to move from the threatening and vertiginous perceptual realm to the safely exorcised sphere of the conceptual, while at the same time concealing this entire movement by a dialectical dance of complacency. No, change is involved with suffering not because of change per se but because things do not remain the way we wish them to remain even when the way we wish them to be is “to be changing.”
