Aquarium Salt, known as AQ salt acts a tonic per say for Betta fish and other fish. It is a refined salt safe for use with freshwater fish.
It is quite commonly used to assist healing torn fins, ich, fin-rot and other external diseases. It should not be used when internal parasites are infecting a betta. AQ salt can also be a perk up, similar to a tannin effect from indian almond leaves.
Do not use AQ salt with live plants, they will suffer and possibly perish. Ideally a 1 gallon hospital tank should be utilised, this assists in monitoring of the betta, and simplified dosing. The ideal hospital tank consists of 1g tank, heater, internal/bobbing thermometer, mug/cup or other small decorations that will not be affected by treatment and can easily be sanitised to remove any lingering diseases.
You can generally purchase AQ salt from a pet store, aquarium or online. Normal salt should never be used with freshwater fish.
Hope this helps!