The Wall Street Journal reports that Nasdaq will drop Apple's profile in its Nasdaq-100 stock index, lowering Apple's share from 20.5 percent to around 12.3 percent, more in line with the number of actual Apple shares out there. Apple is one of 81 companies who are seeing their shares lowered by the rebalance, while 19 other companies, including Google and Microsoft, will have their shares increased. The index was last adjusted in this way back in 1998, but back then, Apple obviously wasn't nearly as big a company financially as it is now, so Nasdaq is simply adjusting things to bring shares more in line with the actual market.
The changes should take effect on May 2, and Nasdaq does say that it expects some trades to happen as a result of the changes, which may lead to some "instability" in the markets for a short period. But in general, the changes are just to make sure that a huge amount of growth (or failure) on Apple's part doesn't upset the index too much.
As always, we are not financial professionals, and any news about AAPL stock should not be taken as financial advice.