On a day barely warm enough to thaw, the speaker notices that gnats have already come back to life. The observation is small, but it leads to a stranger thought: insects are so completely "lifeless" in their dormant state that they are "deader than any corpse." A dead body simply stops. An insect in winter seems to go further, vanishing into a stillness so clean that nothing is left to show it was ever alive.
The last stanza pushes the idea further. Ice, with its "many knives," usually stands for winter's power to destroy, yet here it can't even begin to do its work on the insects. There is nothing for it to cut or crystallize.
The mood is quiet, wondering, and a little eerie. The speaker doesn't seem alarmed so much as amazed, looking at the tiny gnats with real respect. The poem turns an ordinary hint of spring into something close to a miracle, and the title's "Reanimation" gives the gnats' return an almost unsettling, resurrection-like weight.