The world is charged with the grandeur of God.
It will flame out, like shining from shook foil;
It gathers to a greatness, like the ooze of oil
Crushed. Why do men then now not reck his rod?
Generations have trod, have trod, have trod;
This line is the first instance in the poem of Hopkins' notion of  inscape and instress . Inscape refers to the “individually distinctive beauty of style” ( Concise Oxford Companion to English Literature , 340) or the essence of a thing suddenly encountered and understood, like a divine spark. 
 To elucidate what Hopkins means by “foil,” a letter Hopkins wrote to Robert Bridges states: “Shaken goldfoil gives off broad glares like sheet lightning and also, and this is true of nothing else, owing to its zigzag dints and creasings and network of small many cornered facets, a sort of fork lightning too” ( Letters , ed. C.C. Abbott, 1955, p. 169). 
 Metaphorically , when the foil is shaken in just the right way, it gives off sparks that reflect the grandeur and glory of God, the essence of the world charged with his being.