The FTSE 100 Index is a stock market index that represents the 100 largest companies listed on the London Stock Exchange. It is often used as a benchmark for the performance of the UK stock market and includes well-known companies from various sectors, such as finance, energy, and consumer goods. The index is calculated based on the market capitalization of its constituent companies, meaning that larger companies have a greater impact on the index's value. Investors and analysts use the FTSE 100 to gauge market trends and make informed investment decisions.