Hardy's "Where the Picnic Was" depicts a trip by the speaker to the place where a summer picnic once took place. The picnic has literally and figuratively made its mark on the landscape, just as it has on the speaker: Hardy depicts the "burnt circle" which represents the "last relic" of the group who visited the place in the summer. The word "relic" is important here; a relic is something more usually associated with saints, and in his use of it, Hardy is associating the summer excursion with something holy which is now lost to him.
Hardy's poetry frequently revolves around loss, and that is certainly the case here. Although the memory of the summer is "burnt" upon him as it is on the landscape (Hardy is often at one with the landscape in his poetry), it has now passed. Hardy uses understated, quiet language to indicate that one of the people who ventured to the spot with him in the summer is now dead—the manner of her death seems to have been peaceful; she has simply "shut her eyes" as if going to sleep. Her death is not presented as markedly different from the passage of the other two picnic-goers into the "urban roar" of, presumably, London or another city.
Whether by leaving the countryside due to death or simply due to circumstance, the net result is the same: the speaker is isolated, the last survivor of the "band" who once made a fire there. The speaker repeats the word "I," underlining the fact that he is now alone in his travels through a "forsaken" place, where the "winter" of the year seems to echo the winter of the speaker's life. The form of the poem also supports this melancholy interpretation, as the uneven number of lines in each stanza sees the final line standing alone, isolated and unrhymed, unpaired with anything else.