An index is basically an indicator. It gives you a general idea about whether most of the stocks have gone up or most of the stocks have gone down. 

The Sensex is an indicator of all the major companies of the BSE. 

The Nifty is an indicator of all the major companies of the NSE. 

If the Sensex goes up, it means that the prices of the stocks of most of the major companies on the BSE have gone up. If the Sensex goes down, this tells you that the stock price of most of the major stocks on the BSE have gone down. 

Just like the Sensex represents the top stocks of the BSE, the Nifty represents the top stocks of the NSE. 

Two types of Index

Nifty
Sensex

Nifty - Nifty consist of a group of 50 shares. 
Sensex - Sensex consist of a group of 30 shares