Why do the electron in Bohr's principal quantum levels or ground state do not emit radiation? Bohr said that only certain orbits of definite energy are allowed inside the atom. He said that the electrons in their ground state do not emit radiation and that they will emit radiation when they fall from higher energy levels to lower energy levels. My question is what does orbits with definite energy mean? and why do the electron in their ground state not emit radiation?
 A: 
what does orbits with definite energy mean?

It means that each orbit has an amount of energy associated with it. If you move between 2 orbits, it requires a certain amount of energy. It will be the same amount of energy for the same transition in the same atoms. Different transitions have different amounts of energy and different atoms have different orbits (resulting in different energies).

why do the electron in their ground state not emit radiation?

The ground state is defined as when all the electrons are in the lowest energy configuration for the atom (or ion). Electrons emit radiation (photons) when they drop down in energy levels in the atom. If the electrons are in the ground state, they cannot drop lower in energy, thus emitting a photon.
A: 
My question is what does orbits with definite energy mean? 

It means that electron circumscribes an ellipse and the total energy of the system nucleus + electron is constant; when the electron is closer to the nucleus, its potential energy is lower and the kinetic energy is higher, but their sum remains the same.

why do the electron in their ground state not emit radiation?

Bohr thought that atom radiating implies that the atom loses total energy and will quickly collapse (nanoseconds). Since by experience he knew it does not usually collapse in that way, he wanted a model where the atom would not radiate - then, in his line of thinking, the atom would not lose energy and the motion of the electron would be stable in longterm.
He probably did not realize that atom can radiate without losing energy on average. Antennae do this all the time.
