Greek-ing Out
Keats was awfully fond of allusions, especially allusions to Greek mythology. In "Ode on Indolence,he gives a hearty shout-out to Phidias, a famous Grecian sculptor and artist. The figures that visit the speaker appear "strange" to the speaker, as he/Keats was far more accustomed to the style of Greek figures depicted by Phidias in his giant sculptures. The figures he sees now are, instead, more like the figures you'd see on a Greek urn: in profile, walking one after the other as if in procession. These figures haunt the speaker throughout the poem.
Greek stuff is a popular topic with Keats, actually. But Grecian urn-figures aren't the only Greek references in Keats's work. His poems are full of them. We guess you could say the guy was a big fan.