Hello Matty. There is absolutely no need to apologize for asking a question. I'll give you a little background before I explain what a fibrillation is. Thousands of individual muscle cells, together make-up an entire muscle (e.g. your calf muscle). The muscle cells generate electrical activity by being stimulated by nerves that connect and control them. When this electrical activity is generated in the muscle cells, they contract and therefore allow your muscle to contract. Under normal conditions at rest, these muscle cells do not generate any electrical activity (i.e. when nerves aren't stimulating them, they do not generate electrical activity). However, when nerves lose contact with the muscle cells, the muscle cells begin to spontaneously generate electrical activity on their own. This is what is known as denervation (i.e. nerves losing contact with the muscle cells) and it can happen for a gazillion different reasons.