If you search on 'stabilizer' you will find tons of posts, so take a read of at least some of them to get an idea of what these things are and the benefits.
Your engine/trans is rubber mounted in the frame, to isolate engine vibration and give you a comfortable time! The swingarm and rear wheel is mounted off the rear of the transmission case. Rubber has a degree of 'give' in it, so to support the mounts Harley fit two stabilizers to your bike. One is in front of the motor, the other between the cylinder heads. Buells and rubber mount Sportsters have a third near the swingarm axle.
The 'stabilizer' we are talking of here is a third one for touring bikes, which fits under the trans and supports the rear mounts, like a Buell and Sportster. There are several brands of these things, so you must decide which to get. I have True-Track kits on both my Glide and Dyna, which is my recommendation. I don't have any connection with them, just a very happy customer.