What is an Efficient Market?
The simplest definition of market efficiency is that the price already reflects the available information and thus buying or selling the stock should, on average, return you only a “fair” measure of return (after transaction costs) for the associated risk.
On average you will make money, but the money you make is just enough to offset the risks you have assumed.
There are three basic types of information efficiency. Strong form, Semi-strong form, and Weak form.