During a physiological response (whether it be sweating, increasing the heart rate, or thinking), our cells need to talk to each other, in order that they may act in a coordinated fashion. Hormones and neurotransmitters are examples of signal molecules that our cells use to communicate with one another. This communication is so important, that most of the known drugs (and poisons) are molecules that either enhance, or block, the action of signal molecules.
This is a really “key concept” in physiology, so the idea of a “lock and key” provides an especially fitting metaphor. Just like a door key, the signal molecule has a structure that must fit precisely into the receptor (the “lock”) in order to activate it. Some drugs, known as agonists, are designed to mimic (or even exceed) the effect of the signal molecule, by having a similar enough structure that they are able to bind and activate the receptor — essentially “picking the lock”.
Other drugs, known as antagonists, have a similar enough structure to bind to the receptor, but fail to activate it. These drugs prevent the signal molecule from gaining access to the receptor, and thus prevent the normal response from occurring — essentially “jamming the lock”, which is what would happen if you were foolish enough to try picking it with a twig.