A gas at low pressure and high temperature will produce emission lines. A gas at low pressure in front of a hot continuum causes absorption lines. All objects radiate and receive energy. In everyday life, we call this heat. The hotter an object, the more energy it will give off.
All objects have internal energy manifested by the microscopic motions of particles.
There is a continuum of energy levels associated with these motions. If the object is in thermal equilibrium , it can be characterized by a single quantity, it's temperature. An object in thermal equilibrium emits energy at all wavelengths. A black object or blackbody absorbs all light which hits it. This blackbody also emits thermal radiation.