This poem of course expresses Keats' belief in the way that "the poetry of the earth" never dies, no matter how inhospitable the conditions of the seasons, as in the heat of summer, the grasshopper is able to continue the song of earth's poetry, and then passes the baton on to the cricket in the depths of winter, who is able to continue the tune.
The first phrase you have indicated is used to describe the grasshopper and the sound that it makes as the rest of nature are languishing in the heat of mid-summer. Note how this scene is described:
When all the birds are faint with the hot sun,
And hide in cooling trees, a voice will run
From hedge to hedge about the new-mown mead;
That is the Grasshopper's--he takes the lead
In summer luxury,--he had never done
With his delights; for when tired out with fun
He rests at ease beneath some pleasant weed.
The grasshopper then is presented as singing the "poetry of earth" in every hedge during a scene where other forms of nature are shown to be silent and exhausted. Your second phrase of course is also part of this quote, and seems to point towards the playful and joyful way in which the grasshopper captures the delights of summer in his actions, enjoying and capturing the decadence of summer in his voice and the way he rests "at ease" when he is tired of his "fun."