Don't worry it's a really simple question.
In manga and anime you often hear people complain about having trouble reading something someone wrote, normally in the form of a letter or something like that, but the reason they have a problem is that the person has a problem writing either kanji or hiragana and end up leaving it out, and can be hard to read when only one is used.
Anyways...my question is, is it only kanji or hiragana that is written in the letter making it hard to read? If you have any idea what I'm talking about and know the answer, please let me know.