A brick is a workout usually done by duathletes or triathletes where they do one sport after the other. Some people like to swim then bike or bike then run. Usually its in the same order as the sport will be on race day.
Now in the midst of my training I have began realizing that brick workouts are the mainstay or the very key component in triathlon. Kinda like how long runs are the lifeline of doing a successful half marathon or marathon, the same goes with brick and triathlons. For some time I have been neglecting this part of my training and only been doing it once every other week or so. Now though that I have finally finished marathon training, I'm coming around to doing a full time triathlete training once again. I have a hard time mixing the two, now I can finally focus on that one event.