A thiol is a molecule that has a R–SH group. Thiols are like alcohols with an oxygen atom changed with a sulfur one. They have often a very strong and bad smell. Thiols are added to natural gas so that people can smell it. This is because methane does not have a smell but can explode very easily.
Thiols are very easy to oxidize. They are also quite acidic, more than alcohols. When the proton is taken away by a base, the thiol can become a nucleophile. It can do reactions like nucleophilic substitution.