Non-words (also spelt nonwords) are tokens which do not start with a letter of the alphabet. Examples of non-words are numbers, punctuation but also tokens such as 25-hour, 16-year-old, !mportant, 3D. Tokens such as post-1945, mp3 or CO2 are normal words because they start with a letter.
(There might be rare cases when the corpus author used a different definition in their corpus. The definition is part of the corpus configuration file.)