The speaker is pleading with a woman, and it isn't working. In the first stanza, his prayers land on her ear like a fine frost: light, cold, and quick to melt or be brushed away. They make no impression. The second stanza turns bitter. He calls her response ingratitude, and he describes his own words as "the poor words of a liar." That phrase is tricky. He may be admitting he has lied, or he may be quoting what she thinks of him. Either way, his words have lost their worth.
The last stanza suggests the title's meaning. Words that should have stayed private have been "expressed in rust," as if something intimate came out corroded and ugly. Judging by the imagery, the speaker regrets saying too much, or saying it badly. The mood is cold, brittle, and a little ashamed, with frost and rust both suggesting decay and distance. The poem's brevity makes the feeling sharper: he has spoken, and nothing warm came back.