"Tax rulings" are individual decisions by tax authorities which inform the tax payer. These instruments are a kind of "comfort letter" sent by the authorities to companies or individual citizens on tax matters.
The EU says it has "serious doubts" about the compatibility of these particular rulings decisions with the Treaty rules on state aid. They aren't questioning "tax rulings" in general, but rather the particular decisions for Apple, Starbucks, and Fiat.
It is well known that some multinationals are using tax planning strategies to reduce their global tax burden. These aggressive tax planning practices erode the tax bases in our Member States. In particular some multinationals attempt to reduce the profits they declare in countries where taxes are higher. This is mainly achieved through transfer prices, namely the prices charged for commercial transactions between entities which are part of the same corporate group.
The EU believes that in the case of Apple, the national tax authorities renounced to tax part of their revenues by allowing them to lower their taxable profits.
The opening of in-depth investigations is based on a preliminary analysis and before making a decision, the EU says it will look carefully at the comments by the Member States concerned and all interested third parties who want to send in comments.