The Hunter × Hunter Alphabet is a simple substitution cipher for Japanese.
The alphabet in the world of Hunter × Hunter is made up of runic looking symbols. In reality these symbols are simple substitutions for Japanese kana, the collective term for hiragana and katakana; two of the three written systems in Japan.
In other words, a specific character in the Hunter × Hunter alphabet corresponds to a specific kana in Japanese. This in extension means that all the signs and written media seen throughout the World of Hunter × Hunter are real Japanese words and sentences, and can thus be rendered readable in all cases, provided that one knows the cipher and understand Japanese Kana.
The characters used in the cypher were created by Yoshihiro Togashi, the mangaka.