Healthy hearts beat at a regular rhythm. Sometimes Heart arrhythmia may occur, in which the heart might beat too early/fast (tachycardia), too slow/late (bradycardia), or just become irregular. More information can be found at the MayoClinic regarding Heart Arrhythmia conditions.
The IHB is merely an indication that a heart beat did not fall at the regular time, not whether it was fast, slow or random. It can be used in conjunction with other biometrics to assist clinicians.