The S&P MidCap 400 e-mini futures are futures contracts on the S&P MidCap 400 index. Effectively, they are a way to invest in the S&P MidCap 400. These contracts traded on the Chicago Mercantile Exchange. These contracts are traded 24 hours a day on the CME Globex exchange.
The notional value of each contract is US$100 times the value of the S&P MidCap 400 stock index, which means that an investor holding one of these contract will profit (or lose) $100 for every 1 point change on the S&P MidCap 400 index. When the S&P MidCap 400 is at 550, each of these contracts is worth $100*550 = $55,000. An investor does not have to pay the entire amount to hold these contracts; CME guidelines suggest that an investor needs anywhere between 5%-20% as collateral for these contracts.
Unlike ETFs, such as the SPDR funds, the e-mini futures are not backed by stocks. Hence an investor does not get the dividends on the underlying stocks. Instead, these contracts are backed by a counter-party who takes the opposite position on these futures. E-mini contracts are also offered on other indices, notably the NASDAQ 100 and the S&P 500