Someone is being told to sit in a sunken stairwell, a low, sheltered spot, and to "rejoin" something there. The speaker doesn't say what, but the poem suggests a kind of return to oneself, or to a feeling once known. What gathers the listener together is not comfort but "real physical cold." The speaker treats that sharpness as useful, because it makes the present moment impossible to ignore.
The tone is quiet and almost instructive, like a calm voice giving advice. Tomorrow is left to the "plain sun," so the poem asks us not to worry about the future. There is also an admission that "there is no place / to refuge from," which hints at some trouble that can't be escaped. Still, the central idea is gentle: warmth survives only in a moment cold enough to make it matter. The ending admits loss ("We have lost most of"), yet the cold moment redeems it. The mood is stark, reflective, and oddly consoling.