What is a tardigrade "in tun form"?
From Illinois Wesleyan University:


  One way in which tardigrades have adapted to various types of
  environments, has been to reversibly suspend their metabolism. This
  state is known as cryptobiosis [ 'hidden life' (Clegg, 2001) ] and is a truly deathlike state.
  Metabolism lowers to 0.01% of normal or is entirely undetectable and
  the water content of the body decreases to less than 1%. [...]
  
  The most intensely studied type of cryptobiosis is anhydrobiosis, a
  form of cryptobiosis initiated by desiccation. [...] Anhydrobiosis is an almost complete
  loss of body water and the animal can stay in this state for an
  extended period of time. Tun formation, a vital part of the process [Fig. 1.],
  results in a body that is constricted and folded. The first step is
  invagination of the limbs, longitudinal contraction of the body, and
  infolding of the intersegmental cuticle. Wax extrusion covers the
  surface and may help to reduce transpiration (water loss by
  evaporation). The tun formation process requires active metabolism. [...]



Fig. 1. Tardigrade before and after entering the tun state. Note the decreased body length and infoldings. Source: Media &amp; Society Journal. 

Reference
- Clegg, Comp Biochem Physiol B Biochem Mol Biol (2001); 128(4): 613-24