What does 'separate from them in bed' mean, and for how long is it permitted?

Hajr in an-Nisa' 4:34 means the husband not sleeping with his wife, or turning away from her in bed, as a sign of displeasure, in the hope that she will reconcile.
It is done inside the home, not by leaving the wife outside or shaming her in front of others; the Prophet ﷺ advised "do not shun her except within the house" (narrated by Abu Dawud, hasan sahih). The texts set no fixed period; scholars say it should last only as long as needed to have an effect. The Prophet ﷺ once kept away from his wives for a month. Shafi'i scholars distinguish: separating beds is permitted for the purpose of correction, but not speaking for more than three days is forbidden. It is not a tool for punishment or torment. See Nushuz & conflict.