All molecules absorb and emit radiation. Energy can be stored in a molecule in a number of ways: for example, an electron in the molecule might get kicked in to an excited state, or the molecule might start to vibrate in a certain way, or it may start to rotate. The surprising thing is that all of these energy levels are quantized, meaning that for any of these modes, the molecule will only absorb (and emit) photons of a specific series of energies. These energies are called "spectral lines".
For any given type of molecule, we can determine exactly what the freqencies are for its spectral lines (and their relative intensities). If we want to find that type of molecule in a distant source, we need only look for its spectral lines in the light coming from the object. (Usually you would look for absorption lines that are created as the light of a background star shines through a foreground molecular cloud.) While it is possible that some other molecule might have a spectral line that overlaps one you are looking for, it won't fool you if you examine enough lines. No two molecules share exactly the same series of spectral lines; it's rather like a fingerprint.
In this case the interesting spectral lines are in the radio bands, but this sort of analysis can also work with visible light. On a clear night, see whether you can find the planet Mars. To your naked eye, Mars will appear reddish. The redness is from the presence of iron oxide.