“Fartlek” is a Swedish term for "speed play" & entails changing speeds & paces at random points in the run. These pace changes can be regulated or not & are designed to mix up a runner’s pacing clock.
First, by changing speeds at unexpected times, you force your body to listen to YOUR directions, rather than vice versa. Secondly, you begin to get a better idea of your race or comfort pace when you change things up. Your comfort pace is the pace at which you are comfortable holding for the duration of a 10K. You are trying to train your internal clock to run at YOUR effort.
You will run as fast as you want for as long as you want & then rest & repeat. You should change up the speed & length on each fast section, & you should change the amount of rest as well. This is a very free form workout that teaches marathoners and half marathoners strength & discipline. Initially, many runners dislike fartlek workouts for that exact reason, but as they become more experienced with their ability to determine pace & effort they find the format fun & challenging.
The goal is to vary the pace & time of the running & recovery. Practice listening to your body, & remember that the faster you run the shorter the duration should be & the longer the recovery.