How does a register remember value? So I am studying this great book, and Chapter $3.1$ is about registers.

Quoting from this book / chapter: 


  A register is a storage device that can "store" or "remember" a value
  over time, implementing the classical storage behaviour $out(t) = out(t-1)$.


What is mean by:


  the classical storage behaviour $out(t) = out(t-1)$ ?


If $out(2) = out(1)$ and $out(1) = out(0)$ how are we ever going to store a value?
Registers can be thought of as collections of flip-flops. These have inputs which, when activated, change the stored value. When not activated, the stored value stays the same. Look at the truth tables in the Wikipedia article.