The question is referring to events that happen in chapter seventeen of Tuck Everlasting. Miles and Winnie are awake early that morning, and Miles offers to take Winnie with him while he fishes for breakfast. Late in the chapter, Miles hooks a fish and pulls it into the boat. Winnie is horrified and amazed at the same time.
Winnie drew up her knees and stared at it. It was beautiful, and horrible too, with gleaming, rainbow-colored scales, and an eye like a marble beginning to dim even as she watched it.
Winnie must be more horrified by the thought of the fish dying than anything else, because she insists that Miles put it back.
The hook was caught in its upper lip, and suddenly Winnie wanted to weep. "Put it back, Miles," she said, her voice dry and harsh. "Put it back right away."
I believe that Winnie asks Miles to put the fish back because she is sympathetic toward the fish. Not only fish, though—Winnie is sympathetic toward all life. She can't bear the thought of anything dying, which is probably why she pours the spring water on the toad late in the book. Despite Winnie's aversion to the necessity of death, she never drinks the spring water herself, which shows readers that she eventually comes to understand the point that both Miles and Angus were trying to make. Life and death are natural.
But dying's part of the wheel, right there next to being born. You can't pick out the pieces you like and leave the rest. Being part of the whole thing, that's the blessing.