Brake modulation how much fine control you have to apply a range of braking power distributed over the pull of your brake levers.

No modulation is basically no braking vs wheel locking. Having low modulation will mean it's hard to feather the brakes and you can only really lock up the pads. Too much modulation means you will bottom out on the levers before you lock up on the wheels. Ideal modulation gives you access to the whole range of braking power desired and allows you to apply it evenly across the range of the brake lever, giving you fine control over feathering the brakes, stopping firmly, or locking the wheel.