Could aliens understand human movies?

Probably—but not automatically, and not the way humans do.

Aliens could understand parts of human movies if they had enough context: our languages, body language, biology, social behavior, storytelling conventions, and the technology needed to decode the files or images. Some elements would be easier: motion, faces, danger, tools, landscapes, music patterns, and repeated cause-and-effect. Other parts would be much harder: jokes, romance norms, religion, politics, idioms, genre tropes, symbolism, and references to human history.

A silent nature documentary or a simple action scene would likely be easier to interpret than a courtroom drama, satire, romantic comedy, or surreal art film. Even on Earth, people from different cultures can miss meanings in films without shared context—aliens would face that problem multiplied by biology and cognition.

So the best answer is: aliens might decode what is happening, but fully understanding why it matters to humans would require a lot of cultural translation.