Slipspace field pod

Slipspace Field Pods are the Forerunner equivalent of the UNSC cryotubes. Instead of freezing a being in stasis they put the user into slipspace where time stops.

Forerunner cryotubes are somehow connected to slipspace and normal space, the actual bodies are within slipspace, making them safe from nearly everything. (Nearly means that they might be able to get killed if they are engaged within slipspace, otherwise, normal space can not hurt the person inside.)

They are visible via an afterimage that is somehow also portable, but they are at the same time in slipspace.

On the sides of the pods there are Forerunner icons which Dr. Catherine Halsey translated into "That which must be protected . . . behind the sharpened edge of the shield . . . beyond the reach of the swords . . . for the reclaimed", Halo: Ghosts of Onyx, page 337

Theory behind the Technology
Being in a Slipspace Field Pod can be compared to getting sucked into a black hole or what scientists think would happen if that were to occur. The gravity of the black hole distorts time the closer you get, at a certain point, stopping time for you (although a black hole singularity would probably kill you), and if another person was watching, they would see you frozen in time. You are visibly there, but you are not, you are stuck in a vacuum of time, which might be what causes those afterimages from the pods in Halo: Ghosts of Onyx.