In Chapter 18 of The Scarlet Letter, Hester Prynne and Arthur Dimmesdale decide that they will end their years of silent suffering and run away, together, from their Puritan community.

This decision has an enormously positive effect on the emotional state of each of the lovers.

About Dimmesdale, Hawthorne writes:

a glow of strange enjoymnent threw its flickering brightness over the trouble of his breast...His spirit rose...and attained a nearer prospect of the sky...

Hester removes the scarlet letter from her dress and flings it into the forest; she also removes the "formal cap that confined her hair." 

her youth, and the whole richness of her beauty, came back from what men call the irrevocable past...and [she experienced] a happiness before unknown...

At this point, Hawthorne describes (perhaps in an exaggerated fashion) how the happiness of the two lovers spread beyond themselves and affected the natural world around them:

as with a sudden smile of heaven, forth burst the sunshine, pouring a very flood into the obscure forest, gladdening each green leaf...Such was the sympathy of Nature...with the bliss of these two spirits!

This sentence is the basis for the chapter's name, "A Flood of Sunshine."  It refers to: a)the literal flood of light from the sun, and b)to the overflowing joy that was experienced by Hester and Dimmesdale.

Enotes summary provides some very real analysis to the idea of sunshine, so don't avoid skipping the analysis below.

I find the irony of the title of the chapter beautiful. "Flood of sunshine" is oxymoronic, just like the relationship between a priest and a sinner. The two don't mix, but in this chapter the truth is uncovered between the minister and Hester. Flood usually refers to rain, but means much or abundant. Here I think Hester is hoping for much or abundant positive (which the sunshine is usually attributed to, the good in life). But once again, floods and sunshine don't mix... except for at one time...

Think about the Puritan culture, it focused mostly on their sin and following the rules of the bible. What's interesting about  this title is that according to their beliefs, the only place sunshine and rain (or flood) mix is at the point of a rainbow. The rainbow is a symbol of God's forgiveness of Man's sin, saying that He (God) will never flood the world again. At least, that would be what Puritans believed. Hester has dealt with the sin forever, and in this moment, it is really not all her fault... with her partner in crime, but also in grace.