This movie is a pinnacle in film history for pushing the envelope in the direction of experimental, surrealist filmmaking and imagery. It was much more shocking and confusing for the audience of the day than it is today. This is Bunuel early on. He is given a lot of credit in creating the surrealist movement. This film accompanies dada and dali and the movement of artists towards abstraction and away from literal representation.
While the film is a bit hard to understand that is precisely the point. It is not direct, it does not have an overall universal meaning. It is a metaphor. The film's truth exists in the eyes of the beholder. That's part of the point of the surrealist, abstract nature of it. This film is an extension of the greater experiment of what was then the nascent new artform of a cinema which throughout most of the 1920's was still considered much of a novelty of the sorts that brought it to worldwide attention at the beginning of the century.
Ever since the Lumiere Brothers showed off a train rapidly approaching the screen and scared the hell out of the audience out of their fear that a real train was coming towards them cinema had been slowly experimenting with it's own groundbreaking ability to literally project images of reality onto a screen and have the audience suspend their disbelief and earnestly think that what they were witnessing was a true unfolding story. German expressionism and Soviet Montage were the most clear examples of how the 1920's was still a period within which filmmakers were also inventors and experimenting with this new form. Dali and Bunuel continued this trend by pushing even further down the rabbit hole of associative imagery, disconnected imagery and re-creating dreams onscreen for audiences as opposed to strict narratives. This film is a product of a time period that embraced the innovation and experimentation of filmmakers rather than tried to confine them to tried and true cinematic convention.