It's a muzzle brake, in my limited experience the most obnoxious known to man. It reduces recoil but I wold refuse to fire a BOSS equipped rifle without hearing protection, even while hunting.
It is also designed to allow you to tune the vibrations of the individual barrel to increase accuracy. If you don't handload, this is perhaps a desirable feature.
Also, in fairness, the BOSS has an optional replacement part that replaces the muzzle brake ports with a solid piece. This of course negates the recoil reduction and reduces muzzle blast to that of a conventional barrel.
A few years ago the BOSS was in style for a while but seems to have faded away from what I have noticed.
Personally, I have no interest in it. It is perhaps 3" long, and if I am going to carry a barrel that long it will consist of BARREL, which increases velocity. With the BOSS Browning has to reduce the barrel length to maintain similar overall lengths.