What does an input string of epsilon mean?

I am currently reading Introduction to the Theory of Computation (Sipser), and after introducing epsilon labeled transition arrows, the book shows the following NFA:

I was following it until I read the following :

Practice with it to satisfy yourself that it accepts the strings ϵ, a, baba and baa...

What does an input string of ϵ mean?

• It's the empty string, so a string of length zero. I'm sure the text explains it, just read it carefully.
– Juho
Mar 3, 2014 at 18:19
• @Juho - You're right - it is explained... Just a little hidden away, took me quite a while to find it - thanks
– jcw
Mar 3, 2014 at 18:37

In the context of NFAs, $\epsilon$ marks state transitions that do not consume input. These transitions thus express the non-determinism of the automaton.
When discussing the acceptance of $\epsilon$, the symbol marks the empty string (this is equivalent to the condition that an accepting state is reachable by a sequence of $\epsilon$-transitions from the start state). to avoid confusion, some authors use a different symbol for the empty string (often $\lambda$).