Abjad is a type of writing system where each symbol represents a single sound or letter in a language. It's kind of like the alphabet you learn at school, but instead of having symbol sound combinations for each letter, there's just a symbol for each sound.
So, for example, in English, the letter "A" could be represented by a symbol in abjad since "A" always makes the same sound. Abjad is used in a lot of languages, including Arabic and Hebrew.
So, if you were learning to write in Arabic using abjad, you would have a symbol for each sound in the Arabic language, and you'd use those symbols to write out words and sentences. It might seem tricky at first, but once you get the hang of it, it can be a really efficient way to write and communicate.