The phrase "empty jar" means a jar or container that has nothing inside it. It is often used to describe something that lacks substance, value, or significance. Full definition
I love using an almost empty jar of almond butter and the smaller mason jars. (kimscravings.com)
The dressing can easily be made in your almost empty jar. (naturalgreenmom.com)
People find more appeal in things that seem scarce: They say a cookie tastes better if it comes from a near - empty jar rather than a full one. (scientificamerican.com)