Fibrillation is an irregular and rapid contraction or shaking of the heart muscles. It causes the heart to beat in an uncoordinated way [when things are not working together in a smooth way], which can lead to a decrease in blood flow to the body.
For example, if someone is experiencing fibrillation, their heart may beat very fast and in a chaotic pattern, which can make them feel lightheaded or short of breath.
Very Simple Definition:
Fibrillation is when the heart beats in a fast and messy way. It can make you feel dizzy or out of breath.
For example, if your heart is fibrillating, it may feel like it’s fluttering or pounding really fast.