Haven't seen the movie, but it's a rather good novel by W. Somerset Maugham. I don't recall if there was any direct reference to what it meant in the novel, but I'm sure it must be a allusion to Shelley's poem, which the book mirrors to a great extent:
Lift not the painted veil which those who live
Call Life: though unreal shapes be pictured there,
And it but mimic all we would believe
With colours idly spread,--behind, lurk Fear
And Hope, twin Destinies; who ever weave
Their shadows, o'er the chasm, sightless and drear.
I knew one who had lifted it--he sought,
For his lost heart was tender, things to love,
But found them not, alas! nor was there aught
The world contains, the which he could approve.
Through the unheeding many he did move,
A splendour among shadows, a bright blot
Upon this gloomy scene, a Spirit that strove
For truth, and like the Preacher found it not.
Interestingly, it wasn't the poem the poem that inspired the story, it was these lines of Dante:
'Pray, when you are returned to the world, and rested from the long journey,' followed the third spirit on the second, 'remember me, who am Pia. Siena made me, Maremma unmade me: this he knowns who after bethrothal espoused me with his ring."
Maugham wrote a good foreword to the novel where he explains this in some detail.