The Brake Bias (found on the "Mechanical Grip" page of the cars setup) allows you to control the distribution of braking effort between the front and rear brakes. It can also be used to manage brake temperatures (although on a fairly minimal level) by moving the distribution forwards or rewards, depending on where you need more or less brake heat.
The value shown in the car setup is the percentage of the front brakes.
Setting the Brake Bias towards the front of the car (higher value) will provide added stability under braking, but will increase the chances of locking a front tyre and increase understeer when simultaneously turning and braking.
Moving the Brake Bias rewards (lower values) can help maximise braking on all four tyres and increase rotation when trail braking and turning into the corner. However, going too far can destabilise the car under braking, particularly if the rear tyres lock up first.
Ideally you want to set the Brake Bias as far rewards as possible and is comfortable, allowing you to brake deeper into the corner and use the brakes (through trail braking) to rotate the car upon corner entry without losing the rear of the car when coming off the brakes and turning in. Some cars allow a greater rewards bias than others.
Pro Tip: In wet conditions, set the Brake Bias as rewards as possible / is comfortable. This will reduce front locking and reduce the understeer in these lower grip conditions.