ASCII stands for the American Standard Code for Information Interchange, and it's the most common way to encode text characters in a byte. The columns are the first hex digit, the rows are the second, so 0x41 is 'A', and 0x39 is '9'. Just to be clear, 0x20 is a space character.
ASCII dates back to the 1960s when teletype machines (a combined keyboard and printer) were in use. The first two columns are sometimes known as control characters. They didn't print anything, but were used to control the printer (e.g. 0x0C, "FF", was "form feed": move to a new page).
If you've ever wondered why your keyboard has a "Ctrl" key, now you know. Holding it down while typing a letter moves you a few columns left into the control characters.
<Ctrl>-C gives you ETX (end of text) which has been a popular way to terminate programs. ArdEx continues this tradition.