A heterosexual (or straight) person is sexually attracted to a gender/sex that is different than their own, and is usually limited to one gender. A male liking a female and a female liking a male, as an example. Can reproduce (under most circumstances) with each other, and is considered by some the "default" or normal "sexuality", although this notion ties in with heterosexism. Heterosexuality is a very common sexuality.
The word "heterosexual" comes from the Greek heteros meaning different or other, as a prefix for sexuality.