A false document is something that someone makes or changes to trick others into thinking it is real. It's like when you pretend to be a superhero and make a paper mask to wear, but you're not really Spiderman.
Some grown-ups make false documents, like fake IDs, to try and buy things they aren't old enough for or get into places they shouldn't be. Others might change a real document, like a diploma, to make it look like they graduated from a school that they didn't actually attend.
It's important to know that making or using false documents is against the law and can get people in trouble. It's always better to be honest and truthful!