An index is a basket of securities whose combined value is tracked as a single number. S&P Dow Jones Indices defines it as a representative sampling of a market segment: the S&P 500 follows five hundred large US companies, the Nasdaq 100 the largest non-financial names on that exchange, the DAX the forty biggest listed in Germany. Move the basket, and the level moves with it.
You cannot buy an index directly, so the brokerage account quotes a contract on it, usually built on the index futures. US500, NAS100, GER40: the ticker is the broker's, the price comes from the futures. One contract gives exposure to the whole market in one position, which is why indices are the natural home for strategies that trade risk on and risk off rather than a single company's news.
Indices have session hours that matter. The cash market for US indices is open from 9:30 to 16:00 New York time, and that is when volume and volatility concentrate, but the contracts trade nearly around the clock at thinner liquidity. Economic data at 8:30 New York time and the cash open itself are the moments most index strategies are built around, or built to avoid.
A strategy tagged Indices on Javlot trades these contracts. The DAX and the Nasdaq are the most common, and both are known for fast, deep intraday moves.