The Financial Times Stock Exchange 100 (FTSE 100, or the Footsie) is a share index representing an average of share prices in the 100 largest, most actively traded companies on the London Stock Exchange.
The FTSE 100 measures the daily share price performance of those 100 firms.
As the Financial Times explains, if the FTSE 100 is up, it means there are more people buying than selling and share prices have risen. Conversely, if more people are dumping shares the index goes down.
The index began on 3 January 1984 at the base level of 1000 and is maintained by the FTSE Group, today's wholly owned subsidiary of the London Stock Exchange which originated as a joint venture between the Financial Times and the London Stock Exchange.
The highest closing value reached to date is 7037.67 (with an intra-day high of 7065.08), on 23 March 2015.