It is described as "hologramatic exercises for the dead", and Rimmer does them aboard Red Dwarf as part of his daily achievement list. Rimmer often reminded the human Dave Lister that he conducted exercises despite the fact he was dead, to push the point home to Lister how slobby he was.
Rimmer is seen doing these exercises throughout the earlier series, especially Series I. Rimmer named them "Necrobics" in the episode "Confidence and Paranoia", and Rimmer had also got the skutters to place a designated Necrobics chart above the mirror in the original sleeping quarters (possibly brought in from the gym), which can be seen throughout most of Series I.
When Rimmer was faced with his own hologram clone, one of the consequences was they pushed each other in Necrobics to breaking point, since holograms still feel (albeit simulated) fatigue. (Me²)
The rogue computer Queeg took over Rimmer's body and forced him to do his "regulation exercises" to the full, and beyond. Queeg forced to Rimmer to go on runs for miles, despite the fact Rimmer had already passed out unconscious at the beginning of the run. ("Queeg", Series II)
Rimmer seems to have given up on exercises after this incident, although he partaked in the Necrobics deck and Sexual Recreation decks when he temporarily joined the crew of the hologram ship Enlightenment. (Holoship)