"But that dread of something after death, --- The undiscover'd country, from whose bourne no traveler returns, --- puzzles the will..."

Hamlet, Act III, Scene I

This is from Hamlet's infamous soliloquy about suicide, where the toubled Prince of Denmark backs away from the awful deed (for the time being) out of fear of what will come next.

"The undiscover'd country" refers to a future, of a sort, but more accurately it refers to death.