How does a register remember value?

Question

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?