It means that the **domain** of the function is $[0,1]$ and that the **codomain** of the function is $[0,1]$. An example of such a function would be $x^2$, for example, or maybe $\sin(x)$ or an infinitely many other functions.
To solve your problem, here's a couple guidelines:
Take a look at the function $g(x)=f(x)-x$.
1. What is the sign of $g(0)$?
2. What is the sign of $g(1)$?
3. What does the intermediate value theorem tell you?