[Unix; common] To bring a task to the top of one's stack for immediate processing, and hackers often use it in this sense for non-computer tasks. "If your presentation is due next week, I guess I'd better foreground writing up the design document."
Technically, on a time-sharing system, a task executing in foreground is one able to accept input from and return output to the user; oppose background. Nowadays this term is primarily associated with Unix, but it appears first to have been used in this sense on OS/360. Normally, there is only one foreground task per terminal (or terminal window); having multiple processes simultaneously reading the keyboard is a good way to lose.