This is dry ice, frozen CO2 gas, solid Carbon Dioxide. On earth it converts from a solid directly into a gas without becoming a liquid. The process is very much like a liquid evaporating into a gas except its a solid evaporating directly into a gas while skipping the liquid phase.
This process is called sublimation. Sublimation is an endothermic reaction (absorbs energy) meaning energy has to be present for sublimation to occur.
Nitrogen gas, which we breath on earth, is a solid toothpaste consistent ice on Pluto and it sublimates when it warms to the correct temperature/pressure.
Sublimated solid nitrogen (along with carbon monoxide and Methane) becomes Pluto's atmosphere.
We see the effects of sublimation on Pluto at Sputnik Planitia.
The air you breath on earth is 78% nitrogen as a gas but nitrogen on Pluto is frozen solid except when it has just enough energy to sublimate into a gas. The sublimation process at Sputnik Planum looks like inverted hollow half shelled bubbles.
Look at these hollow half spheres or cups they look like popped inverted bubbles and may be the result of heat rising from below or Pluto's climate.
Certainly there must be some form of viscous relaxation taking place in order for objects to impact the surface then disappear sorta like quick sand swallowing an object except in this case it's more like slow sand or as Alan Stern describes it the nitrogen ocean has a toothpaste consistency.
In this image you can see how there are troughs creating cells. To suggest that viscous relaxation is the cause of this surface is to ignore these troughs. Some process is creating them.
If you look close at this image one thing that seems to be taking place regularly is the more aggressive looking sublimation cups seem to cluster around the trough's edges. while further away from the troughs the sublimation cups tend to lighten up significantly.
The more sublimation taking place the more likely it is these areas are warmer than the areas with less sublimation cups.
The central parts of the cells are not smooth but the sublimation process appears to be significantly subdued compared to the zones around the troughs.
Sometimes an area without a clearly defined trough will have aggressive sublimation pits which seem to be turning into a trough but not quite there yet.
Sometimes the heat that makes the sublimation pits is strong enough such that the surface near the most aggressive pit formation becomes completely smoothed over. In other words, the fluid is so warm it can't even retain the shape of the sub cup.
More or Less
This is the general idea.
Areas with more pits tend to line up near areas where troughs exist or seem to be about to form.
It seems as though the aggressive action of the pits precede the formation of the troughs.
This suggest heat is welling up at these spots which in turn forms the troughs.
In other words, the troughs are the byproduct of their alignment with a source of heat from below indicated by the sublimation cup's size and quantity.