E-Minis are “mini” futures contracts. These are futures contracts small enough for an individual investor to trade. Two very popular E-Minis are the “ES” series and the “ND” series, representing futures on the S&P 500 index and the NASDAQ 100 index, respectively.
Many investors trade these instruments, so they are very liquid. Even more people monitor these instruments, to help them trade normal stocks. At a high level, the E-Minis track the prices of the underlying indices. However, some people see these as leading indicators of the market. This is especially true in the 30 minutes before the NYSE opens, because the E-Minis trade 24 hours a day, and the actual stocks on the NYSE barely trade at all before the opening bell.