The manubrium sterni (MS) is vaguely octahedral bone of the sternum. I know, boring. Well, if you stay tuned, the third paragraph will tell you how to find and explore your MS.
The sternum consists of three parts. The body of sternum (corpus sterni) is the long central plate that most of the true ribs attach to. It sits directly in the center of your chest. At the bottom it grows a little spike, called the xiphoid process. The manubrium sterni sits on top of the corpus sterni. It's a flat plate standing on edge, with five edges forming junctions with other bones. It is often called simply the manubrium, although there are other manubria in the body.
Let's take a guided tour. Start at the base of your throat, and move down. Push in firmly with your fingers, some of the structures we'll be looking at are well below the surface. You should feel a U of bone in the bottom center of your neck where it meets your chest. The very base of this U is the top of the MS, and is called the suprasternal notch1. The sides of this U are the ends of the clavicle bones. The clavicles rest on the two angles sloping down from the notch; these angles are called the clavicle articular facets or clavicular notches. Keep palpitating -- moving down, you may be able to feel where the first ribs join the sides of the MS, at the first rib articular facet. The sides of the MS then angle down to the base, where it joins with the sternum at the manubriosternal junction2. It may take you a bit to find this bottom joint, but if you place your fingers on your mid-sternum and move up you should be able to find a slight dip down as you move onto the MS.
The MS doesn't do much but it does have a lot of things connected to it. The ribs are connected with costal cartilage joints, in the same way as are all other ribs to the rest of the sternum. The clavical joint is a standard synovial joint, of the saddle-type; this joint has to be able to move freely as the trunk and shoulders move. The MS is connected to the corpus sterni with a fibrocartilaginous disc at the manubriosternal joint; this joint allows for a small amount of movement that may be helpful during respiration, although fusion of this joint doesn't seem to cause any significant breathing difficulties.
At the front (anterior surface) two muscles attach to the MS, the pectoralis major and the sternocleidomastoideus. At the back (posterior surface) the sternohyoideus and sternothyreoideus are attached.
Footnotes:
1. AKA the jugular notch.
2. AKA the sternal angle, AKA the Angle of Louis, AKA the manubriosternal angle. Manubriosternal is sometimes also spelled 'manubrosternal'.