The US Dollar Index is very important
in this current market because the cheap dollar makes it inexpensive for foreign investors to buy US stocks -- as the dollar goes up, they will not buy, thus causing the US stock market to go down.
In addition to that, if the dollar continues to go up, those US companies which have products with strong foreign demand will likely experience weakness in future sales (thus hurting future profits) as foreigners revert to holding back purchases of US goods until the dollar returns to its longer term trend which is downward.