Someone sympathetic toward a certain point of view without being a fully paid-up member of the club.
In its literal meaning 'fellow traveller' just means someone who travels with you. It was first applied to non-communists who were inclined toward the views of the Communist Party by Leon Trotsky. He used the Russian word popútchik to indicate that. The term 'fellow traveller' in this sense came rather later, in the New York publication Nation, 1936:
"The new phenomenon is the fellow-traveler. The term has a Russian background and means someone who does not accept all your aims but has enough in common with you to accompany you in a comradely fashion part of the way. In this campaign both Mr. Landon and Mr. Roosevelt have acquired fellow-travelers."