Exposure in photography is in regard to the amount of light that reaches the sensor. its hard to expose a photo properly in extreme light situations, you tend to have an over exposed or underexposed photo. You can control the exposure of a camera in many ways, one of which being the shutter speed. the faster the shutter the shorter the sensor will be exposed to light, the less the exposure. The inverse of this is also true. Another popular way to control the exposure is by turning up the ISO sensitivity. This is a dicey game for the more you do this the lesser and lesser quality your photographs will be.
If you mess up the exposure when shooting, it is possible to fix this by going into a photo editing software to manipulate it, but at this point is significantly more work and its supremely easier to do it right the first time.