Brackish water is water that has a variable salt content between fully saltwater (marine) and fresh.
There are many fish that come from brackish habitats and their bodies have evolved to live in such conditions, hence they must be given brackish water in the aquarium.
In nature brackish water occurs where rivers meet the sea and the two bodies of water intermix. This 'mixing' area continues in some cases for many tens and even hundreds of miles upriver, creating large brackish habitats.
Mangroves occur around tropical regions along vast areas of brackish, fresh, and saltwater and are an important habitat for many animals and tropical fish.
The salt level, or salinity, of water can be measured in 'specific gravity' (s.g.) via the use of a hydrometer, which is used by marine fishkeepers and is easily available. Because adding salt to water makes it denser, water with salt is heavier; Specific gravity is a measure of that weight set against freshwater. Saltwater is maintained at an s.g. of around 1.022-1.026 whilst freshwater has an s.g. of 1.000.
For most brackish fish, an s.g. of between 1.005-1.015 is normally acceptable. Whilst many plants will not cope with high salt levels, some species such as Anubias, Java fern, Hygrophila, Saggittaria, and Vallisneria will cope with water between 1.005-1.010.
Unlike marine fish, which require specific care and extra equipment, brackish fish require exactly the same care as most tropical freshwater fish.