A B-RAS is simply a broadband convergence point - there are several dotted about the country usually in large metropolitan areas - think of it as like a 'super-exchange' where all the broadband connections from each exchange meet up.
The RAS / B-RAS (both terms are generally used to mean the same thing in UK BT Broadband lingo), is made up of some obviously pretty sophisticated equipment (supposedly!). One of the functions of this equipment is to store information about the speed your line syncs at.
This is known as your speed profile, and since the work is done by the BRAS, it's called your BRAS profile! This speed profile restricts the maximum speed you can download at, and is rounded down to the nearest 500kbps below your actual sync speed.
It's not really any more complex than that.
Now obviously, the details of how the profiles work... and importantly how the BRAS profile is rounded down, and how it is updated when your sync speed changes, are a bit more complex, and there's no point me repeating what Kitz has already written on her site, but hopefully that gives you some background as to exactly WHAT a BRAS profile means.
Hope that explains things a bit?