A thioester is a molecule with a group C-S-CO-C. They are like esters, but with a sulfur atom instead of an oxygen one. Like esters, they can be made from carboxylic acids. In the synthesis, a thiol is used instead of an alcohol.
Thioesters are very important in biochemistry. They are made in the body during reactions that make fatty acids. They are also important intermediates in the production of ATP, which is the molecule that gives energy to the body.