You’re moving the braking pressure toward the front or rear of the car. Too much front bias and you lose braking power since you’re not using the rear brakes fully, and you also risk locking the fronts when braking hard or trail-braking. Too much rear bias and you lose braking power since you’re not using the front brakes fully, and you risk locking the rears when braking hard or trail-braking.
You want it balanced enough that you’re using all four corners while still being controllable.
Generally, more front bias is going to be easier to drive consistently because it’s easy to see the front tires start to lock and adjust your brake pressure accordingly. It’s harder to run more rear bias because if a rear tire locks, you’re in for a ride pretty quickly. Much harder to catch and save. Plus, it’s easy for the rears to get light on turn-in if you are trail-braking.
iRacing sets most cars up to be a little more front biased so it’s easier to drive, but you can definitely go faster and brake later if you can learn to control the car and master your threshold braking with a little more rear bias. Many of the cars in-game have the ability to control bias on the fly as well. Bind that adjustment to some buttons/keys and go out for a practice session, get the tires up to temp and slowly move the bias rearward until it becomes un-drivable. This can be an important adjustment throughout a run as well, as tires fade and fuel goes down.
In GT4 for example, I’m almost always starting my brake bias at one point more forward and once the tires get hot, I start moving bias backward in 0.5-1% increments until I get it to the limit of what’s drivable.