The Salic law was the civil law code started by Clovis, the first King of France. It called for agnatic succession (only males could inherit) of the throne.
It had not been a relevant part of French law. In 1317 Philippe V used it as justification for taking the throne after the death of his brother who had only a daughter as heir. Philippe had no male heirs so his brother succeeded him. When he too died without a male heir, their sister Isabelle claimed the throne on behalf of her son, Edward III of England.
The Salic Law became the justification for France refusing his claim. It was the start of the Hundred Years War.
