A stronger bottom
In the seated glute stretch, the target is your Gluteus Maximus or to put it in layman's terms - your bum! The glute muscles are the largest in your body and if they are tight then it can have negative effects on your running (lower back pain after a run is one example), it is therefore a good idea to stretch them out and strengthen them as part of your running regime. This stretch is one that can be easily carried out wherever you are and will help to strengthen your glute muscles and increase your flexibility.
On floor method
You should feel this stretch in the large muscle that joins your bottom to your thigh.
Sit on the floor or a mat with legs straight out in front of you, cross one leg over the thigh of the other. Lean back slightly and bend the knee of your lower leg so that just your heel is on the floor. Grasp the upper foot with one hand and the knee with the other. Lean forward by pulling your torso toward your legs. Hold the stretch for at least 20 seconds and do not bounce. Repeat with opposite leg.
As you continue with this stretch you will find it becomes easier and your muscles will not be getting the benefits they once did. By tilting your pelvis upwards and/or bringing your outstretched leg closer (heel closer/knee higher) you will intensify the stretch and give your muscles the work out they require.