First of all, it is important to know that Ancient Greek pottery was often decorated with very beautiful and intricate scenes of ancient Greek life. When Keats refers to people, such as the Bold Lover, he is speaking to the painted figures.
Below is an example of a Grecian urn (though not the one that inspired Keats!):
The poem is all about eternity and eternal things.
In the first stanza, he is contemplating the vase in its entirety. He marvels at the piece's perfection (still unravish'd bride of quietness) despite its age (foster child of Silence and slow Time). He then feigns ignorance of Greek culture to reinforce this, asking "What men or gods are these?" etc. Basically he is making the point that so much time has passed that whole cultures have vanished yet this urn has survived unscathed. If you really think about the 2600 years that have passed since pots of this type were made in Ancient Greece, and all the people who have lived and died since then, and all the wars that have been fought and all the destruction that has ravaged the world, then you will begin to see what astonished Keats. That despite all of that something so fragile and detailed and beautiful produced by a skilled hand remained unspoiled and beautiful. This is the sense of awe with which the poem must be understood.
In the second stanza, he is speaking to the individual painted figures. At first to a lute player. Though the lute player is not real and cannot really play (being a painted figure), Keats envies him the eternity which he will spend lost in his song (most educated people could play some sort of instruments back when Keats lived so he and his readers would have known the joy of being lost within music while playing it). The lute player is evidently playing beneath some trees which Keats says will never lose their leaves. Again, he is envying the fact that the figure will get to enjoy an eternal summer. He then speaks to another figure. A young man trying to kiss a young woman. And he realizes that the young man will have to spend eternity tantalizingly close to the girl he desires and yet will never be able to reach her with his lips. But again he tells the young lover not to grieve because the girl he loves will never grow old. She will always be young and beautiful and he will have eternity to gaze upon her. And he says that though he will never have the bliss of kissing her, he will still have undying love.
The third stanza talks about all of those issues again and elaborates on them.
The fourth stanza describes another scene on the pot of a religious ceremony. A heifer is a cow, and in this scene it will be sacrificed as part of the ceremony. And now he considers which Greek town it was that so long ago put on this ceremony, which the urn is a record of. Again you get the feel of the amount of time that has passed. You know as Keats knew that when the urn's painter painted this scene, he must have had specific local events in his mind which he was representing. And then you think of how many days have gone by since that ceremony. And then you feel the sense of awe that Keats felt when confronted by the mind-boggling age of the urn. Today this loses its meaning somewhat because we all have learned about dinosaurs which lived over 65 MILLION years ago and the Big Bang which happened something like 10 BILLION years ago. In comparison 2600 years seems small, but you haver to remember that when Keats wrote most people thought the Earth was only a few thousand years old. The important point is that the urn is very old compared to any person or even existing civilization.
The final stanza, like the first one speaks more generally again. He is again saying that though other things will decay (When old age shall this generation waste) the urn will remain and will always confront those who care to listen to it with that sense of eternity and awe which Keats describes with his most famous lines:
'Beauty is truth, truth beauty,—that is all
Ye know on earth, and all ye need to know.'
It is important to know that Keats was sick most of his life and died young. Death is very much shunned by modern society and we do not think of it all that much. We are sold the notion of eternal youth in advertising all the time. Just consider any commercial for make-up or shampoo etc. But in Keats's day life was much harder and often much shorter. This seemed to make people far more aware of death. So the sight of this deathless, eternal piece of beauty must have been even more stunning.