The speaker describes two small discoveries. In the first, they come across their own heart in a box among old clothes. It's a strange, almost funny image, but it also feels tender: the heart has been packed away with things that no longer fit or get worn, as if the speaker had stored their feelings and forgotten them. Finding it again suggests a quiet reunion with a part of themselves they'd set aside.
The second discovery is a feather, found "at the end of an infinite day." That phrase suggests exhaustion, a day that dragged on and on. Against that heaviness, the feather is light and unexpected, a small gift that appears just when it's needed. The title, "Foundlings," ties the two moments together, since a foundling is something found and taken in. The mood is gentle and a little wistful, with a hint of relief. The poem doesn't explain itself, which leaves room to feel the odd comfort of finding something lost.