Something that's alien is different or foreign. In science fiction, space aliens come from other planets. In reality, illegal aliens come from other countries.
When we talk about aliens, or things that are alien, we're talking about things and people that are different and strange to us. Like an alien from outer space. Some people are afraid of illegal aliens from other countries. Besides people, experiences can be alien. If you've never owned a cat, then having a cat is alien to you. You can also alienate people by making them feel weird, disconnected, and alone: like an alien.