GH is measured in dGH (degrees of GH) and ppm. It measures the amount of calcium and magnesium ions in the water. In other words, how hard or soft your water is. 

It is an indication on whether your aquarium water has enough minerals(Magnesium, calcium etc.) and/or salts that are essential for health and growth of your fishes and even plants. 

Ideally, freshwater aquariums have a GH between 4-8 dGH (or 70-140 ppm).
A change in GH levels may also be required when breeding certain fishes such as Discus.

Some of the common symptoms of low GH include,

• Fishes with poor appetite, slow growth rate, lethargy, or faded colors
• Plants with signs of calcium or mineral deficiencies
• Crayfishes, shrimps having trouble molting, death during molting
• Snails with thin, flaking, or pitted shells

Instructions:

• Add 5ml of aquarium water in to the test vial.
• Add GH reagent by the droplets. Swirl for about 5 seconds after each drop.
• Stop when the color changes from red/pink to blue. Count the number of drops.

Each drop of GH reagent corresponds to 1 dGH unit.
For planted freshwater aquaria, a GH value of 8 – 12 dGH is advised.

Note:
Keep out of the reach of children! Only for aquarium hobby use.
The use of protective gloves and eye protection is recommended.
Please read the instructions carefully before using the product. 

Salifert:
Salifert is a Netherlands base company that produces top of the line test kits as well as tank additives and foods for marine and reef aquariums.
Salifert is well established and popular with hobbyist for their accurate and affordable test kits.