A market index is a score that measures the performance of a group of assets. Major stock indices track a basket of the most important stocks on a certain stock exchange. For example, the NASDAQ 100, one of the leading US indices, consists of 100 stocks traded on the Nasdaq stock exchange.
While you cannot invest in indices directly, with Octa you can trade contracts on them. It means you do not have to buy or sell any actual stocks. Instead, you can make profits by predicting how an index is going to change and opening a corresponding long (Buy) or short (Sell) position.
Trading contracts on indices allows you to use leverage, or borrow funds to open much larger positions. This way, leverage can magnify both potential profits and eventual losses. Indices are relatively intuitive: with them, you trade the direction of a stock market in general, and there are many strategies for that.