A mint mark tells you where the coin was made.
Many coins also have the designer's initials hidden somewhere in the design.
This is not the mint mark. The mint mark is almost always directly on the field and not usually part of any other device. This is because it can affect the rarity of the coin. For example, in the case of 1894 dimes, having the 'S' mint mark adds tens of thousands of dollars to the value of the coin as only 24 were minted. Many American coins lack a mint mark.