While Colin's comment is broadly correct for marine aquaria -- the green section probably corresponding to about 1.022 to 1.026 -- for brackish water aquaria things are different.
To start with: understand what a hydrometer does. It shows the density of water (usually called the "specific gravity" or "SG"). Freshwater has a density of 1, usually written as
1.000; normal seawater at tropical temperatures has a density of
1.025. In other words, seawater is 2.5% more dense than freshwater.
With me so far? Good. Now, brackish water contains less salt than seawater. So the density is going to be less than 1.025. If you're keeping "low end" brackish water fish, i.e., species that need just very slightly saline conditions, you're likely going to need around 10-25% the salinity of normal seawater. In density terms, this comes out (at tropical temperatures) to 1.001 to 1.005.
High salinity brackish water fish need a salinity around 40-50% that of the sea. In density terms, that's around 1.009 to 1.012.
Now, when you look at your hydrometer you will see that it will be marked off in units. Typically these are one "point" on the density scale. So when you put the hydrometer in plain tap water it will read 1.000. Each mark on the scale is one point upwards, i.e., 1.001, 1.002, 1.003, and so on.
To explore the relationship between salinity, density (specific gravity) and temperature, download my freebie Mac/Win program
Brack Calc. As you'll see, moving the temperature alters the density of the water, which is why you must make the brackish water up with lukewarm water rather than cold water.
For most practical purposes, you can estimate the amount of salt you need by weight, and then check the final density using the hydrometer, adding salt or water as required. Do always remember that salt absorbs water from the atmosphere, so unless you keep it in an airtight container, after a while any given weight of the stuff will result in lower than expected salinity because part of that weight was water from the air.
Cheers, Neale