The speaker imagines their own tomb being mended by summer air, as if warmth and breeze could do the work of a mason. It's an odd, quietly startling picture: death is not shown as something frightening but as a place being made ready, even made pleasant.
The second stanza reinforces this idea of safety. The tomb will have "every safe doubt and redoubt," a phrase that plays on defensive fortifications (a redoubt is a stronghold) while also hinting at doubts, the uncertainties we carry. Those doubts seem to be softened or sheltered rather than solved. In the last lines, the evening itself is "spare / like a bed": plain, uncluttered, and restful.
The mood is calm, a little wry, and strangely tender. The speaker seems to feel acceptance, even ease, about mortality. The poem suggests that thinking about the grave can be less grim than peaceful, like looking forward to sleep at the end of a long summer day.