The poem describes a small, everyday moment: someone checks the mailbox and finds a key. That key isn't for the house or the mailbox itself. It opens another box, one that holds a package. So the speaker has to take one more step before getting whatever has arrived.
The poem never says how the speaker feels, which is part of its charm. Judging by the wording, though, there's a quiet sense of anticipation. The word "waits" makes the key seem patient, almost alive, as if it knows something is coming. The package stays out of sight, so we're left curious about what it might be.
The mood is calm and a little mysterious. It takes something ordinary and shows how a simple chore can feel like the first step in a small treasure hunt. The three short lines mirror this, giving us just enough to wonder, then stopping before we find out.