A Stasis Field is a name for both a device and the effect it causes. Essentially a time-warp generator, stasis fields create breaks in the normal flow of space-time within their area of effect, slowing massively the progress of causality. The closer one is to the centre of the stasis-field, the slower time appears to move; a single moment can take so long to play out inside a stasis field that it appears frozen to observers outside the effected area. Because the contents of a stasis field are divorced from the normal passage of time, it is effectively impossible to alter the conditions of the inside of the field in any way. This makes whatever is caught in the area of effect almost completely frozen and unable to be harmed.
Stasis fields are normally used to contain people and objects in effectively unchanging conditions. Possibly the most famous stasis field is that which encloses the body of the Ultramarines Primarch Roboute Guilliman, who was interred in a stasis field upon the moment of death. Eversor Assassins are kept in stasis chambers when not active in the field.