The Financial Times Stock Exchange 100 share index; an average of share prices in the 100 largest, most actively traded companies on the London Stock Exchange. 
The FTSE 100 - or the Footsie, as you will sometimes see it written and pronounced - is an index that measures the performance of the shares of the 100 largest companies listed on the London Stock Exchange, sometimes refered to as the LSE. It measures the daily share price performance of those 100 firms.
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. There are several other indices, such as the FTSE 250, and each one provides a quick snapshot as to whether people are making or losing money.