Mason jars are made for canning food.   You don't want that food to spoil.   You seal the jar by sterilizing the jar, and by putting hot food in it and putting the seal on, then screw the lid on.  As the contents cool, the air inside will contract causing the lid to seal tight.  Your jar of food will be good for months or maybe even years.   It's what people did before there were grocery stores to store food for their families.