Wet dreams are when an individual orgasms in their sleep due to a dreams which may or may not be erotic. This happens involuntarily and is not caused by masturbation. Wet dreams are experienced by both people with vulva and penis, but there are people who don't experience wet dreams at all. Wet dreams are more common to people with penis as they ejaculate in their sleep which wakes them up with wet cloths and bedding. Wet dreams are extremely common and there is nothing wrong in having them. They do not reduce an individuals sperm count or immunity nor are they a sign of any illness. The medical term for wet dreams is nocturnal emission.
Sources:
https://www.medicalnewstoday.com/articles/321351#facts-and-myths
https://teens.webmd.com/wet-dream-faq
https://www.healthline.com/health/healthy-sex/wet-dreams
https://www.verywellhealth.com/what-is-a-wet-dream-and-what-does-it-mean-3015090