The BRM is part of the DBS (just like the ESS) that exists in NS and if the BRM is low you get less ISK.
BRM = bounty risk modifier, which modifies how much of the ISK is paid out of a bounty
DBS = dynamic bounty system, which exists in null sec systems. It combines BRM with the ESS to modify bounty payout based on how much risk it is to get bounties in that system (risk is based on how many rats are killed, players are killed and whether people come to steal your stuff
ESS = encounter surveillance system, takes some of your isk initially but pays a large part of that out after some time if it does not get stolen, which encourages people not to dock up and wait until hostiles are gone, but to fight for your ISK (or to forfeit it).
Another smaller part goes to the reserve bank, which provides value to the low sec sites where you can get the keys to open this bank
Player 1 is very risk averse and likes killing NPCs while looking netflix. He hides in a faraway system that rarely is visited, together with others doing the same. The high amount of safe ratting results in a low BRM of 50%, which means only half the bounty for rats is paid out, part of which goes to the ESS for some time. In that time, a hostile player comes by, all the ratters dock up and the hostile player steals the value in the ESS.
Result: player 1 spends a lot of time with low risk for very little ISK
Player 2 decides to go ratting in their main null sec system. High activity and regular visits of hostiles means the BRM is very high at 200%.
When hostile ships inevitably show up, he docks up, swaps ships and goes to the ESS to fight for the value that’s inside.
Result: player 2 spends his time split between ratting in a risky system and fighting in the ESS and gets paid much better. If he does not die…
Player 2 is getting paid 4 times as much per NPC, but player 1 can likely kill more rats undisturbed. Player 1 had to pay less attention where player 2 may have several cloaky campers in system at all times and gets many visits from hostile players.
As a result, it’s good to use more expensive ratting ships (marauders, capitals) for a situation like player 1, and if you’re flying a cheap replacable ship (cruisers, battlecruisers) you can take some more risk like player 2.
The DBS allows players to choose their own risk/reward profile while ratting in null sec.