It can tell you which elements a material is made of.
Carbon, oxygen, hydrogen, etc. Each element has a “spectrum.”
Same idea when they look at stars and planets.
Apparently, they had troubles decoding more complicated molecules.
Not exactly. To "decode" a molecule (or other entity), you want the item to remain intact to the maximum extent possible (to get it's "parent mass"). The larger the molecules get, the more places they have available to break apart. If there is too much fragmentation, you lose the ability to extract the structural info.
Yet to get the molecule to work at all, you have to get it into the gas phase, remove at least one electron (to get it charged so the electric and magnetic fields can "steer" it, or accelerate it so that different mass fragments can be detected).
Past methods of ionization involved hitting the vaporizing molecules with a beam of electrons, or charged particles, which can be TOO energetic for the less stable giant molecules. So you need a kinder/gentler ionization means. This is one approach of several.
And the above is a VERY simplistic picture.