Thyme is a popular and very aromatic herb grown in Europe and is related to mint. There are numerous different varieties of thyme. It provides a sweet and earthy flavor to many dishes, and is commonly used in Mediterranean cuisine. Thyme can be used to flavor lamb, pork and game and has the benefit of being able to withstand long cooking times. As a result, it is often used in pasta sauces as well. It is an important ingredient in classic dishes like coq au vin and osso bucco.
As it is a well-known and widely used herb, most serious cooks will keep a bottle on their spice rack. However, you may have run out of thyme or it may just not be something that you thought you would ever need. If you are looking for a substitute, the good news is that (as with most ingredients) there is a multitude of good options.