first of all u need to understand that each atom's got their own "fingerprints" (i.e they will only absorb and emit a certain wavelength of light)
well emmision is basically u heat up an atom and make it emit light (which u can detect by looking for its "finger prints" )
in absorption spectrum ur basically bombarding the atom with all wavelengths of light, and seeing which wavelenght it absorbs (which is its fingerprints btw) , once u know which wavelength was absorbed- then u know what atom it was.
i know my explanation was a bit scrappy- but hope it helps.