Dragonmommy....
The only stupid question is the one that remains unasked.
Brackish water is water that has more salt content than freshwater but not as much as sea water. This kind of environment is typically found in areas where rivers empty into the ocean (estuaries).
Salinity in water is commonly expressed in specific gravity values. Pure water (distilled) has a specific gravity of 1.000. Salt water has a specific gravity that generally ranges between 1.020 - 1.024 (though this varies a bit depending on location). Brackish water typcally has a specific gravity in the 1.002 - 1.008 range so you can see that is lies between pure distilled water and sea water.
There are a number of types of fish that thrive in brackish environments. Some of the more common are mollies, scats, many gobies (including bumblebee gobies and the so-called dragon eel that many large chain pet stores are selling), archer fish, monos, and some catfish like the 'shark' catfish.
Hope this helps a bit,
-Joe