What does it mean if a film is directed by Alan (or Allen) Smithee?

It means the director of the film does not want his/her name associated with the final product because it had developed, out of their control, into something that did not fit with their vision. Sometimes it is also used when a television (or streaming) adaptation differs from the original to an extent the original director finds unacceptable.

The Smithee pseudonym was created for use on the film Death of a Gunfighter, released in 1969. During its filming, lead actor Richard Widmark was unhappy with director Robert Totten and arranged to have him replaced by Don Siegel. Siegel later estimated that he had spent 9 to 10 days filming, while Totten had spent 25 days. Each had roughly an equal amount of footage in Siegel's final edit, but Siegel made clear that Widmark had effectively been in charge the entire time.[2] When the film was finished, Siegel did not want to take the credit for it and Totten refused to take credit in his place. The DGA panel hearing the dispute agreed that the film did not represent either director's creative vision.

https://en.wikipedia.org/wiki/Alan_Smithee