A more thorough explanation:

Propter negligentiam haeredis jus suum non prosequentis is a Latin legal term that means "on account of the negligence of the heir in not following up the heir's right." This term refers to the situation where a vassal's heir fails to enter the estate for a year and a day, resulting in the forfeiture of their right to the land.

For example, if a vassal dies and their heir does not claim the estate within a year and a day, they lose their right to the land. This is because the law assumes that the heir has abandoned their claim due to negligence. In this case, the estate would be passed on to the next rightful heir or revert back to the lord of the land.

This legal principle was commonly used in medieval times to ensure that land was not left unclaimed and unused for extended periods of time. It encouraged heirs to act promptly and claim their inheritance, or risk losing it altogether.