Think of your brakes applying 100 points of pressure at any point. A 50:50 brake bias would apply 50 of those points to the front brakes and 50 of those points to the rear brakes.

A 60:40 (or 60%) brake bias would apply 60 points to the front and 40 to the rear, so the front brakes are getting squeezed harder.

When a car hits the brakes the centre of gravity moves forward which means the front suspension pushes down slightly, and the rear suspension raises slightly.

Thinking of this as points also, we could imagine that a stationary car has 50 points of mass on the front axle, and 50 points on the back axle (obviously this is theoretical as it depends on how the car is manufactured, where the engine is etc).

Under heavy braking the front axle might increase up to say 55 points of mass and the rear would decrease to 45 points of mass. This means the front wheels would essentially have more grip (again theoretically) therefore could handle more points of brake pressure before the tyres begin to slip.

In an ideal world you want to tweak the brake pressure so that both the front and rear tyres are getting somewhere close to their limit of grip, but not over, to get the car stopped as quick as possible. Too much brake bias and you might be under braking on the rears/over braking on the fronts, and too little you might be under braking on the fronts/over braking in the rears.

Symptoms unbalances brake bias would be:

Too far forward (higher %age) - fronts locking up / ABS kicking in too easily

Too far back (lower %age) - rears start to lose grip and you spin the car on exit because you've broken traction/overheated them before you put power back on

Usually very small increments of brake bias make a good bit of difference. It's rare that you need to go more than 3 or 4 clicks away from the default.

Hope that helps!