Barebacking is when people have sexual intercourse without using a condom. The term originated in the gay community, and was used to refer to unprotected anal sex. The word is related to bareback, which means riding a horse without a saddle. Barebacking refers to a deliberate decision to not use a condom, forgetting to use a condom is not barebacking.
Barebacking is dangerous, because there is a high risk of infection with sexually transmitted diseases, such as Syphilis, or HIV. Even if both partners are HIV positive, there is a risk of infection with other strains of HIV. This may mean that the disease becomes more difficult to treat, or that certain drugs stop working.