The VIX is an index which measures expected volatility on the S&P500 for the next 30 days. It can be traded like an asset on the Chicago Board Options Exchange. It is calculated from the trading of options for the next 30 days. For example, if a large amount of calls and puts with exercise prices a long way away from the current market prices, then the expectation is that there will be volatility and the VIX will be high.
Generally speaking a VIX higher than 30 implies high volatility whilst a VIX below 20 implies stability.