A braking mass is used in braking system, it is very technical. To make it simple to understand, let me briefly describe it as follows:
If it is for a braking device. In its metal friction body, it has an internal cavity that is filled partially with substances that have high capacity for temporarily storing heat to absorb heat from the friction during braking operation. This substance is call "braking mass".
By the way, such a term can also be used in shooting range in knocking down septum for bullet, or in a sew cleaning device. You only ask "the braking mass". So I tell you the most common application. If you can give me the information where you get this term, I MIGHT be able to tell you in more detail.