Exposure is one of the key components in
photography. It means "the total amount of light that reaches the film".
Exposure is almost always controlled by manipulating the f-stop and the shutter speed.
The exception is flash photography, where the shutter speed does not change much: it is just the f-stop.
What does good exposure mean ? Well, the definition is quite relative, it all depends on what your aims (and you materials) are.
For negative materials, any printable negative could be considered "well exposed". For slides, you would require a certain contrast range. If you practice the zone system, on the other hand, you have an almost scientific definition of exposure.
On a personal side, for me exposure is the photographic moment: it is when I decide, arbitrarily, to chew off a piece of reality, and put it on film. I rob the world of its photons. It is an act of selection. When I press the shutter release button I don't breathe, and it is not only for practical reasons.