It is the key indicator. A lot of traders and investor keep an eye on it. Here was the result from Tuesday (1.10.2019). The value was at 47.8 really below consensus 50.1. This data started a sellable rally. S&P after this data dropped more than 3 percent (when we count Wednesday too).

Source: FXstreet
Let's look at technical view and possible way. Which levels are important and key for market?

It seems that on S&P 500 is created a bullish trend channel, where are important support and resistant trendlines. Now the support one is closer and is important for another growth. There is rising possibility that the market will test it, where we can also find EMA 200 as the main support (and the previous HIGH too). From this supports, there is a chance of move higher but the market need some reversal pattern f.e. bullish pin-bar, which is known as pattern, which "ends" downtrend. On the other hand when the market breakdown the EMA 200, there is strong green area, where could be a reaction.
That bad data could not be as bad news as market think. The reason is that after bad data, there is rising possibility for another rate cut or QE from fed.
This analysis serves only for educational purpose and can NOT be considered as investing advice.