Literally: able to be travelled after. To be nachvollziehbar is to be such that another person can walk the same path and arrive at the same place.
English translates it as understandable or comprehensible, which loses the whole point. Those words describe a state in the listener. Nachvollziehbar describes a property of the reasoning — it can be retraced.
An argument that persuades but cannot be retraced has failed.