Pansexual is a sexual orientation (not to be confused with a gender), which means it has to do with who a person finds sexually attractive.
A pansexual person may be attracted to other people regardless of their gender identities.
This doesn’t mean pansexual people are attracted to everyone, or that pansexual people are more sexually promiscuous than straight people. We still have tastes and preferences in partners. Case in point, I’m pansexual, and I’m very picky.
The key distinction that makes someone pansexual is that gender is not a factor in their attraction. The people that a pansexual person finds themself attracted to may be of any gender, and that isn’t of particular relevance to the sense of attraction.
The prefix “pan” comes from Greek, and means “all.”
Pansexual isn’t the same as bisexual, though those orientations are often grouped together. It’s also distinct from omnisexual. I wrote an article breaking down the differences here if you’d like to learn more.