What is the actual meaning of the lines given in the band structure diagram? For example, the $E=f(k)$ relation for a simple 1-dimensional periodic lattice is $E(k) = -2\cos(k)$.

![](https://i.stack.imgur.com/kzvgd.jpg)
1. What is the physical meaning of this graph? It is showing the energy of a particle at a specific k-point. Does it mean that when our particle is moving in the k-vector direction, it has to change its energy?
2. In this case, there is only one band (only one line). In some other examples, I have seen there are many more bands. What does more than one band mean?