A rebus is commonly defined as a pictorial depiction of a word or a name and was very common in heraldry to denote the surnames of the gentry. The composition of the rebus alludes to the name, profession or personal characteristics of the owner. It is derived from the Latin expression, Non verbis, sed rebus, which means "not by words, but by things".
Phillis Wheatley's poem is a depiction of the images shown on different rebuses she may have viewed. Each stanza refers to a particular pictorial denotation and each reference may have been symbolic of the family or estate so depicted on the rebus.
Stanza one describes a rebus depicting 'a delicious bird' which could be a reference to some kind of fowl which was common in the area and served at the table. The fact that it could have fed an army suggests its prevalence. The bird could also allude to the name of whomever had the rebus constructed.
"Sent by an hand unseen" could have a dual meaning: either that the speaker is expressing a Christian belief in that the 'hand unseen' is a reference to God the provider, or that the actual creator (artist) of the rebus was unknown.
The rest of the stanza mentions a creature of a horned race (which may, for example be a unicorn, which was quite popular) as depicted on British royal standards against a green background - probably indicating lush, green fields - a symbol of prosperity and growth, further emphasised by 'gem' which depicts something of value. One can thus assume that this rebus is the representation of a prosperous family with an important name, somehow related to British hierarchy.
The rebus mentioned in stanza two alludes to a happy town inhabited by vivacious and athletic young men and women. They are 'gallant' which suggests bravery and nobility. Also depicted is a "Dardan hero" who achieved fame for his beauty and liveliness, whose memory is preserved in tales of legend. He was unfortunately vanquished by a king. The rebus, in this regard, is probably one for a town or village that the speaker may have visited. The poet had been in England for example, and could have been impressed by what she had seen there and decided to record her memory in this poem.
The final stanza makes reference to a rebus denoting the initials of a well-loved 'peer' - a knight in shining armour, as it were, who came to rescue a town and restore order - freeing them from abuse and oppression. His heroic deeds have added to the glory and fame of 'Britannia' - another name for The British Empire. The town's name is probably a derivative of his name and title and the rebus preserves his name and deeds for posterity.
It is clear from the poem that the speaker derived much pleasure from viewing and interpreting the rebuses that she encountered. Her tone is quite positive and it is clear that she admired the British, which is quite ironic since she was a slave. That however, can only be properly understood if one delves into her history.