It is private or confidential information that confers an advantage in insider training, and the person who sells or buys stocks on the basis of that private information makes money he or she should not be entitled to make. The idea is that stocks and bonds are traded on a level playing field, with buyers and sellers all having access to the same information, making their decisions on the basis of information that is available to everyone. This keeps the stock market fair and allows it to be a legitimate measure of value.
Let us suppose I work for a technology company that is on the verge of some major technological breakthrough, for instance, the ability to hold millions of gigabytes of memory on some device no bigger than a dot. This is not public information, and it is likely to be confidential information, which I have agreed to keep confidential as a condition of employment, a fairly standard requirement. The stock of my company is so-so, perhaps trading at $25.00/share. I call you up and tell you about this breakthrough. You buy 100 shares of the stock, investing $2500. The announcement of the breakthrough comes, and the shares are now trading at $50.00/share. You can now sell this stock at the current rate and will have doubled your money.
The problem is that you have reaped the benefit of information that is not accessible to everyone, just to you, because I have called you to tell you about the breakthrough coming. Not only that, but also, I have breached my employment contract, discussing confidential information with an outsider. This is completely unfair to all the buyers and sellers in the stock market, since the whole market is premised on the idea of the level playing field. If insider training were not prohibited, our ability to trade and make money would be based more on whom we know than on any acuity we have to judge what to buy and sell. Our faith in the stock market as a true evaluation of the worth of publicly traded companies would be completely eroded, and we would probably all go back to hoarding gold or stuffing our money in our mattresses.