Please stop sniggering while Madame L explains that the word "fartlek" comes from the Swedish and means "speed training." It's a specific training method for runners that combines regular continuous training with interval training.
Madame L reads on the Wikipedia page that while it mostly refers to training for runners (as Madame L always thought of it), it can also be used for training in other sports.
The idea, generally, is for the athlete to have fun, to spend 45 minutes, for example, just running or walking or jogging or doing whatever seems to fit the individual athlete's needs. Madame L tried this when she did more running than she does now, and she loved it. And, almost as important, her running times improved as a result of the fartlek training.