Question

What does negative sign in energy of nth orbit signify?

Answer 1

We say that a free, unbound electron has zero energy (that's convention, you could just as well put another number there). This means that the level n=∞n=∞ is fixed at E∞=0eVE∞=0eV. Since the other levels lie lower, i.e. possess less energy, this forces all other bound states to have negative energies - which then represent that we need to add energy to make the bound state free, which corresponds to raising its energy to zero.