Is it true that electrons are arranged in different energy levels in an atom?

It simply means that electrons can only have certain energies. These "levels" are often in terms of n, such as n1, n2, n3, and so on. Let's say that n1=1000 eV (a unit for energy) and n2=2000 eV. It would be impossible for an electron in an atom to have any energy between those two values. This follows true for any energy level; this can be shown better like this: there can be no electron energy found between nx and nx+1 when the electron is in an atom.