The speaker feels caught between how grand he looks and how ordinary he is. He has "a king's head," a kind of noble or impressive appearance, but underneath he is still "the same plain man," as common as meat in a cook's kitchen. That comparison is blunt and a little funny: no one admires meat in a kitchen, it's just there, waiting to be used.
Then a lady enters the scene, and she keeps him out of the garden, which suggests a place of beauty or privilege he isn't allowed into. In return she gives him a single orange for his "feast." The word "feast" is ironic, since one orange is hardly a banquet. Judging by the title, the poem seems to be poking at ambition and status: the speaker may be reaching upward, but the reward is small. The mood is wry and quietly deflated, with a touch of humility and gentle self-mockery rather than open anger.