Early visual language systems were complex and required knowledge of hundreds of signs and symbols, whereas an alphabet, a set of visual symbols or characters that represent the elementary of a spoken language, require only twenty to thirty easily learned signs.

...an alphabet, a set of visual symbols or characters that represent the elementary sounds of a spoken language, require only twenty to thirty easily learned signs.

Explanation:

An alphabet is comprised of graphemes or letters, which represent a sound unit or phoneme in the spoken language. The letters in an alphabet usually present a specific order. Unlike visual language that uses signs and symbols, alphabets are simpler to remember and to learn. Instead of memorizing hundreds, sometimes even thousands of symbols, countries that have adopted an alphabet can use a small amount of letters to represent all the words used in their languages.