A SEG in prison is basically just a prison inside a prison.
When put in SEG you're in isolation away from other prisoners and removed from general population.
When Prisoners are in SEG they are basically in Solitary Confinement and spend from 23 hours out of every 24 hours per day in the isolation cell.
Being in SEG or Solitary is a very dark and boring place even though prison itself is very boring the SEG part of prison is even more boring as you're not coming out of your prison cell for 23 hours out of the day.