The Doctrine of Signatures was the Medieval Roman Church's version of the law of correspondance and stated, broadly, that The Creator had left all sorts of clues throughout His creation to show what uses each part of it might be put to. This was particularly applicable to herbs and similar things, so that the leaf or flower of the plant might resemble the body part which it was suited to treating. Alternatively, the "signature" might be found in the plant's preferred environment or some aspect of its biology (for example its time of flowering or the time of day at which blossoms opened or closed).
Amazingly1 this sort of thinking also allowed astrology to creep back into Christendom through the back door, based on the idea that some part of an objects signature could be read from the positions of celestial bodies - or, conversely, that an individual's astrological profile could determine which herbs would benefit him (for example, if his solar house happened to be Leo, then herbs that were also connected to that house2 might be beneficial to him).
Where this doctrine has not been entirely discredited, there is usually a case of post hoc non est propter hoc behind it - the resemblance is coincidental and seems to serve more as a memory aid, with other unhelpful or harmful plants with equally valid resemblances being ignored.