While they're waiting to be taken back to England from France along with much of the rest of the British army, Robbie and Nettle run into a gypsy woman who offers them food if they'll capture her escaped pig.
The pig is a symbol of sorts for Robbie himself. The woman practically tells him so after they capture it when she says, "My pig will always remind me of you" (2.296). Additionally, Robbie's mother is associated with gypsies—she tells fortunes (which is a stereotypical gypsy thing to do), and Robbie's father appears to have been Romany, or gypsy (see 1.8.8). Furthermore, the woman loves her pig just as Robbie's mother, Grace, loves him. And just as the woman wants her pig back, Grace is somewhere over there in England, praying for Robbie to come home. (In the film, the encounter with the gypsy woman is cut… and replaced by a sequence in which Robbie has a vision of his mother.)
Robbie, who is delirious, seems to more or less understand the connection between the pig and his own predicament. He insists on capturing the escaped critter out of superstition. As he says, he had once believed that stepping on a crack would cause his mother to die—and he hadn't stepped on a crack and his mother hadn't died.
Since the pig is Robbie, and he saves the pig, we should conclude that Robbie survives. And indeed, in the third part of the book we see that Robbie has survived. But then, in the fourth part, we learn that Briony just made that up, and Robbie actually died. Which brings up the obvious question—What about the pig? Did Robbie not really save the pig? Did the episode with the pig "really" happen?
We hope he saved the pig. Pigs are cute.