It is a prefix that isn’t specific to gender (I don’t know of a particular use in sexuality though that doesn’t mean there isn’t one), but in other uses that I know of, it isn’t used by itself as a descriptor of an aspect of a person’s identity, but as part of some other word. It basically means the opposite of trans (as a prefix, so not just “cisgender means someone that isn’t transgender”, but anywhere that the prefix trans- could be used, for example, when talking about spacecraft visiting the moon, the space farther away from earth than the moon is is sometimes referred to as translunar space, and conversely, the space between the earth and the moon can be called cislunar space). In general, if one is talking about people, especially if it’s just used by itself with nothing else attached, it just refers to everyone other than transgender people.