Two stops at the camera would be one stop at the enlarger.
At least if you find that statement true, you have developed your film to an 0.5 contrast which many people like...
I think this is the most direct explanation, if explanation is what you're looking for.
But since I see that you (the OP) are fairly new to photography, some further explanation is in order. "Contrast" in film or paper essentially describes how fast it responds to changes in light level. A one-stop change doubles (or halves) the light coming through the lens. When this happens, a low-contrast film will have a small change, whereas a high-contrast film will have a large change. If you take these two pieces of film, and measure the difference in how much light they block, you'll probably find that the high-contrast film has much greater, more than double the "light blocking" power, whereas the low-contrast film has much less than double the blocking power.
So the contrast of the film mainly determines how much more light it will block as a result of a one f-stop exposure increase. A number of people mentioned something called "gamma," this is roughly the same thing as film contrast.
The way film contrast is measured is like so: you make a series of exposures on a f-stop based scale - each step essentially doubles (or some other factor) the exposure. (Actually, a "log" scale is used, where each change of 0.30 represents either a doubling or halving of exposure, depending on which direction you are going.) Then you use an instrument called a densitometer (it measures optical density) to read the film. Density is also on a log scale, so it correlates to the original log exposure scale. (For example, a density value = 0 means that 100% of the light passes through; film with density = 0.30 lets 50% of the light through, and density = 0.60 only lets 25% of the light through, etc.
Anyway, we can draw a graph of the film's exposure, using a log scale, vs the film's light-blocking power, also on a log scale (aka "density"). (This is called the "characteristic curve" and is commonly shown on film data sheets. It also varies with development time, etc.). If the resulting curve has a slope = 1 (same as gamma = 1), then doubling of the film exposure WOULD result in the film blocking half of the light. But in normal photography, the film almost always has a lower slope, typically about 0.5 to 0.6.
So in the real world, if you increase film exposure by one f-stop, the typical result is that the light-blocking power of the film increases less, perhaps only by about 2/3 f-stop. This is why people are telling you that the change in film exposure does not correlate exactly to the change in printing exposure.
But WAIT! The plot thickens! The photo papers that we print on typically have a much higher contrast than film. So even though the film has a reduced light-blocking effect (compared to changes in the camera exposure), when we print onto photo paper this "reduced effect" is now exaggerated by the higher-contrast paper, and the combined effect is closer to what you expected to see.
Anyway, the process is fairly complicated, and the result won't always be exactly what you expect. Although it IS very predictable if you have all the details right.
