This expression belongs to Joe, an illiterate, but loving man who shelters Pip as a child from the harsh treatment of his wife, Pip's sister who must raise her little brother since their parents are dead. Pip and Joe have enjoyable, loving times on the marsh and at the forge: "What larks!" Joe remarks on these times.
In the "Second Stage" of the novel while Pip resides in London with Herbert Pocket, Biddy writes on behalf of Joe. She tells Pip that Joe and she wonder what he does
for the love of poor old days...he[Joe] wishes me most particular to write what larks.
But, when Joe visits, Pip is ashamed of the man's awkward behavior and inappropriate appearance. Joe apologizes to Pip, saying that he belongs on the forge; he leaves and Pip wants to make "Joe less ignorant and common" that he might be "worthier" of Pip's society.
Later, when Pip returns to the forge for the burial of his sister
larks sang high above it[the earth] and the light wind strewed it with beautiful shadows of clouds and trees.
This is a symbolic sentence as the larks of innocence and happiness have been shadowed by Pip's materialistic and selfish desires to raise his social status.
Then, in "Stage Three" of the novel, Pip is overcome with a fever and falls ill after trying to help Magwitch escape. When he become conscious, he sees his old friend Joe beside him: "...And when you're well enogh to go out for a ride--what larks!"
This joyous phrase of Joe's evokes the good times, the "good old days of Pip and him together, the hopes for Pip's happiness that Joe has always possessed. "What larks"; what fun, what wholesome joy in those we love, in those who are genuine and unpretentious.