Bit rate
Bit rate is the amount of data per second a station uses for its audio. A higher rate generally allows better sound.
The bit rate of a digital station says how much data per second its audio uses. More data allows the coder to keep more detail, which generally means better sound. Less data saves room.
On DAB+ that room is shared. One multiplex has a fixed capacity, and the broadcaster splits it between programmes. A station with a lot of speech can get by with a lower rate than one that plays complex music. That trade-off is one reason two DAB+ stations on the same radio can sound different.
Broadcasters set the rates themselves and may change them.