A fellow traveller is a person who sympathizes and supports the philosophy, aims or methods of a political party or movement, but has no official ties to that party or movement, and may openly deny supporting it while secretly doing so. The term is most often used of supporters of communist parties in democratic nations, though it has also been applied to supporters of ultra-right-wing parties or independence movements.
In some cases, maintaining fellow traveller status may be more beneficial to a person than official membership, as the person could secretly provide support to the movement while in their current position. For example, a person who is a United States government employee might support communist ideals, but officially supporting such a movement may cause issues with employment (such as loss of a security clearance) which could otherwise benefit the movement (e.g. providing classified material to it). Therefore, the person officially denies supporting the movement, maybe admitting to minor participation (claiming not to have known their true aims of the movement, and swearing to have stopped support once they learned such), but secretly providing support to it.
A fellow traveller is different from a dupe, in that though both are not officially members of a party or movement, the fellow traveller recognizes his/her actions are in support of it, while the dupe does not.