For this poem it was quite unique and had many hidden messages for each stanza.
I believe that the first stanza talks about the panther being in a type of cage because of the words "passing bars" along with "a thousand bars".
The second stanza is probably demonstrating how the panther does not want to be tamed so instead it continues to walk in circles over and over.
As for the third stanza, it seems as if the panther had a visitor which ended the panthers life since it states "An image enters in, rushes down through the tensed, arrested muscles, plunges into the heart and is gone". Does that mean that the panther ended up dying? I'm still not too sure what the last stanza is trying to say. But these are what my analysis are at the moment.