What is the meaning of veil of mystery?

The veil of mystery, when aside is drawn, 

Will show to what end all mankind is born—

Who, even as a blade of grass doth grow,

They pass by progress through each stage below,

And when the spirit leaves the coil of clay 

And enters on its dark mysterious way,

It, for its sins committed in the flesh,

Is purified and from all evils wash’d—

And thus progresses through each sphere above,

And, lastly, reaches eternal realms of Love,

Where, through the Great Redeemer’s precious blood,

Is ever singing praises unto God.

—Spiritual