Crucial to understanding this excellent poem by Emily Dickinson is recognising how it is structured. The poem is divided into three sections, each containing two stanzas, and each section describes a condition of the soul. The first two stanzas suggest constraint, restriction, and a violation of the soul, which seems to be described as if it were almost a kind of rape. The middle two stanzas celebrate the moments when the soul is free from such constraints - the sense of freedom experienced is compared to the feelings of a bee which has been forbidden but finally allowed access to the rose. The last two stanzas describe the moment of recapture, when the soul is again put under restraint and, like an escaped criminal, taken back to meet the "Horror."
Your metaphor comes in the latter section of this poem:
The Soul's retaken moments -
When, Felon led along,
With shackles on the plumed feet,
And staples, in the Song...
This stanza gives us a very visual image to convey the restraint that has been imposed: she is a "Felon led along." The metaphor you have highlighted indicates that the feet which were "plumed", feathered, which emphasises the comparison of the soul as a bird that has just been flying free, are now "shackled," limiting any possibility or hope for freedom.