The title literally is taken from a poem written by Saeb-e-Tabrizi, who was a seventeenth-century Persian poet. In this poem, he uses the phrase "a thousand splendid suns" to describe the beauty of Afghanistan and of its cultural achievements. It is therefore partly ironic that this is the title given to a novel that depicts the complete destruction of Afghanistan in terms of its culture and sophistication. There is little evidence of the beauty that the poem alludes to when the Taliban take over. The title therefore partly highlights the tragedy of what has occurred in Afghanistan by forcing us to remember what it used to be like, as the visit to the giant buddha statues before their destruction demonstrates.
Secondly, however, I think we can also argue that the title draws attention to the way in which the beauty of Aghanistan lives on in the love and sacrificial relationship that Leila and Mariam develop together. In a sense, they are two of the "splendid suns" that the novel refers to, who refuse to yield to despair no matter how difficult and challenging the situations they face become.