A "greenhouse" gas or "greenhouse" effect really has nothing to do with a literal greenhouse.

A molecule can only absorb IR if its vibrational modes cause a change in its dipole moment. No matter how you configure the two nitrogens of N2, it will always have zero dipole. Therefore, N2 is not a greenhouse gas, but if you bend a CO2 molecule, it will have a net dipole.

This means it will absorb IR photons of certain wavelengths and become excited where the wavelengths are associated with vibrational modes of the molecules.

Once a greenhouse gas has absorbed an IR photon it could: transfer the photon's energy to another gas molecule during a collision, relax and re-emit the photon, or lose the energy by combination of collision and emisssion which will result in emission of a photon with a different wavelength than the original photon.

As greenhouse gas concentration increases, re-emitted photons are more likely to be absorbed by another molecule, providing resistance to the energy leaving the earth.