A thiol is a type of chemical compound that typically has a strong, unappealing odor.  One example, ethanethiol, smells like onions, and is the chemical that's added to propane to make it smell.  Otherwise propane is odorless and we wouldn't realize when our propane-filled house was about to blow up.  Another example, methanethiol, smells like rotten cabbage, and is found in everything from decaying plant matter to animal feces.  A final example, butanethiol, smells like a skunk, and is the primary chemical in stink bombs.