Tailism is the tendency to expect the workers and the revolutionaries to be separate entities, with the revolutionary's role being to heighten the contradictions in order for workers to spontaneously perform the revolution, so the revolutionary is actually following in the tail of the worker; it's the opposite of vanguardism, which sees the professional revolutionary as the most advanced workers, whose role is to perform the revolution on behalf of, and as part of, the working class. The vanguard is ahead of the revolution in this model.

The problem with tailism is that if you aren't a worker, and your engagement with the workers is purely by informing them (and with the specific goal of tantalizing them into performing revolutionary action) then you are an outsider with no genuine connection to the workers, who have no reason to trust you and will see through your shallow engagement with them.

E: Just to be specific about what I meant, you shouldn't sugarcoat and distort the truth when talking with other people just for the purpose of baiting them into becoming leftists.

Tail-ism... so a revolutionary party will look at a mass of workers and just assume that they will make the right decisions all on their own while the party is most just sitting back being a spectator.

IF, the mass of workers do something good, the revolutionary party committing a tail-ism might take credit for it.

IF, the mass of workers do something bad, then its all the fault of the workers and not the revolutionary party in any way.

In both situations, this revolutionary party hasn't taken the lead by actually doing any work educating, agitating, or organizing the workers.