Definition
A 'fellow traveler' is someone who sympathizes with or supports a particular political ideology or group without being a formal member. Imagine them as fans of a sports team who aren't actually on the team. They often share similar views and may actively support the cause, but they are not officially affiliated. The term is most frequently used in the context of communism, to describe those who were sympathetic to the Soviet Union or communist parties without being card-carrying members.