In the opening stanza of the poem, the speaker recalls finding the dead body of a deer lying on the edge of a road. The speaker says that when a dead deer is found on this road it is "usually best to roll [it] into the canyon," because otherwise, left where it is, it might cause another car to swerve. If another car were to swerve to avoid the dead deer, then that car might accidentally hit and kill another deer, or somebody in that car might be injured.
In the second and third stanzas, the speaker describes how he dragged the body of the dead deer closer to the edge of the road, intending to roll it into the river and so out of the way of any other cars. The speaker says that as he touched the deer's body he felt that "her side was warm." He then says that he realized that the deer was pregnant and that the unborn fawn was still alive inside of its mother.
In the final two stanzas, the speaker describes the moral dilemma that he faced and the two actions that he might have taken upon discovering that there was a fawn still alive inside of the dead deer. The first possible action would have been to roll the dead deer into the river and thus move it out of the way of any other cars. This would, however, make him directly responsible for the death of the fawn, which would surely have died as its mother rolled down the canyon and into the river. At the same time, however, moving the dead deer would help prevent any future accidents caused by cars swerving to avoid the dead deer.
The second possible action that the speaker might have taken would have been to simply leave the deer where he found it, on the edge of the road. This would have saved him from having to kill the fawn but would have made him perhaps responsible for future accidents caused by swerving cars.
Ultimately the speaker decided to roll the deer into the river and thus lower the risk to any motorists who came after him. The fawn inside of the deer would have died anyway, and so the decision that the speaker took was the right and responsible one to take.