We use the definite article in front of a noun when we believe the listener/reader knows exactly what we are referring to: The Pope is visiting Russia. The moon is very bright tonight. Who is the president of France? This is why we use the definite article with a superlative adjective: He is the tallest boy in the class. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.