Exposure affects the white point, or the brightest areas of the image, down to the midtones (mid-level tones around 128, or 50% in LR).
Blacks adjusts the black point.
The first adjustments you should make to your image are Exposure and Blacks. In most captures you can expect a full dynamic range - brightness values going from pure black to pure white. This is represented in by a histogram that spans the entire width of the graph. (But understand that some captures should will have a wide dynamic range... such as a photo of a snowy field or a dark alley.)
Recovery will attempt to recover blown out highlights - areas of the image that are pure white, or level 255, in at least one channel. If all three color channels are pure white, recovery won't help. But if at least one channel has data below level 255, Recovery will move some of that data into the other channels, effectively lowering the brightness from pure white to some level of very light gray.
This is important because when making a print, areas of pure white will have no ink. And this will be evident in the appearance of the print, mainly in what's known as gloss differential. You can easily see areas that have no ink (or very little ink) vs. those with more ink coverage. So blown out highlights are generally best avoided - except in the case of specular hotspots, which you shouldn't worry about.
Fill light opens up shadow detail. Shadows are dark areas of the image that are not pure black, but are darker than mid-tone. In Lightroom, a higher fill light setting will lighten the dark areas (but not blacks) revealing more detail. Fill light will have a very limited effect on black clipping when set properly.
Both Recovery and Fill Light should be used with restraint because both will decrease contrast in the image. Try cranking up both sliders to see how the image starts to appear flat. If you find that you're using Recovery or Fill Light settings of over 15-20, you have a really bad exposure and should focus your attention on better captures.
