It is basically group sex except for this time, only one of them is the submissive or the bottom. This is usually an event conducted by a group of men so several men would cum on the one gay man awaiting their liquids. The meaning is still almost similar however in porn, it would refer to ejaculation of a man into a woman or a gay man's face. However, these days, the word has transcended its meaning it is now heavily related to sex and pornography.
Bukkake literally means 'get splashed' or being poured water on, it is a Japanese term that used to be associated with the making of udon wherein people pour a cup of water into the noodles.
