There are several factors that can influence the value of the assets in an index and therefore impact the price of the index itself. Some of these factors include:

1. Economic data and indicators: Economic indicators such as inflation, GDP growth, employment figures, and consumer spending can have a significant impact on the value of assets and the price of the index.
2. Company earnings: The earnings reports of the companies that comprise the index can have a major impact on the index's price. Positive earnings reports from companies within the index can lead to increased investor confidence and a rise in the index price, while negative earnings reports can cause the index to decline.
3. Interest rates: Changes in interest rates can also impact the price of an index. Higher interest rates tend to make borrowing more expensive and can reduce corporate earnings, which can lead to a decline in the index price.
4. Political events: Political events such as elections, policy changes, and geopolitical tensions can also impact the price of an index. These events can cause investor uncertainty and lead to increased volatility in the market.
5. Investor sentiment: Finally, investor sentiment and market psychology can also play a role in the price of an index. Positive investor sentiment can lead to increased buying and a rise in the index price, while negative sentiment can lead to selling and a decline in the price.

The most traded indices in the world are:

1. S&P 500: The S&P 500 is a market-cap weighted index of 500 large publicly traded companies in the United States. It is widely regarded as the best indicator of the U.S. equities market and is one of the most widely traded indices in the world.
2. NASDAQ Composite: The NASDAQ Composite is an index of more than 3,000 companies listed on the NASDAQ stock exchange. It includes many tech and growth-oriented companies, and is popular with investors who are interested in those sectors.
3. Dow Jones Industrial Average: The Dow Jones Industrial Average, often referred to simply as "the Dow," is a price-weighted index of 30 large U.S. companies. It is one of the oldest and most widely recognized stock market indices in the world.
4. FTSE 100: The FTSE 100 is a market-cap weighted index of the 100 largest companies listed on the London Stock Exchange. It is the most widely used benchmark for the UK equity market.
5. Nikkei 225: The Nikkei 225 is a price-weighted index of 225 blue-chip companies listed on the Tokyo Stock Exchange. It is the most widely used benchmark for the Japanese equity market.
6. DAX: The DAX is a market-cap weighted index of the 30 largest companies listed on the Frankfurt Stock Exchange in Germany. It is the most widely used benchmark for the German equity market.

These indices are popular with investors because they provide a broad representation of the stock market and can be used to track the performance of different sectors and industries.

Going long on an index means buying the index in the expectation that its value will increase over time. Essentially, an investor buys the index with the hope of selling it at a higher price in the future and making a profit. Going long on an index is a bullish strategy, as the investor is expecting that the market will rise.

On the other hand, going short on an index means selling the index in the expectation that its value will decrease over time. Essentially, an investor borrows the index, sells it at the current market price, and buys it back later at a lower price to return it to the lender, thereby making a profit. Going short on an index is a bearish strategy, as the investor is expecting that the market will decline.

Short selling can be riskier than going long, as there is theoretically no limit to how high a market can go, while it is limited how low it can go. This means that a short seller may be forced to buy back the index at a higher price than they sold it, resulting in a loss.

Both long and short strategies can be used by investors to achieve their investment goals and manage their risk exposure. It is important to note that trading in indices can be complex, and investors should carefully consider their investment objectives, risk tolerance, and financial situation before engaging in these types of trades.