The title points to compline, the last prayer of the day in Christian monastic tradition, and the poem seems to begin there. A bell calls, and the speaker answers by "ringing within," as if the sound sets off a matching vibration inside them rather than sending them anywhere. The call is heard in the body.
Then comes the longer, more surprising line: "how infinitely naked is an ear / to the night." We usually think of nakedness in terms of skin, but here it belongs to the ear, which has no lid and can't close itself against sound. At night, when the dark hides everything else, hearing is what's left, and the speaker feels exposed and attentive at once.
The mood is hushed and a little awed. There's vulnerability in it, but not fear. The poem suggests that listening is a kind of prayer: you stay open, you wait, and you let the night come in.