It is not sufficient to only transcribe what was written on the board. In between writings on the board, the lecturer should be verbally giving explanations, intuition, and other helpful remarks. You must also transcribe these. Yes, you will be busy during the lecture. Finally, after class the lecturer may put some clarifying or additional remarks about the lecture on an email or the course website; these too should go into the scribe notes. The goal is to produce a document that someone who did not attend the class could read and learn from