The tare of a container is its weight when it's empty, which is important to know when you can't weigh something without putting it into something else.
Since you can't measure the weight of, say, olive oil without putting it into a container, you need to find out how much the container weighs, its tare, so you don't add that to the weight of the olive oil. The same thing goes for cargo containers, trucks, airplanes, railroad cars. In chemistry, a tare is a counterweight used on this same principle. Tare is also a kind of weedy grass that grows in grain fields or that's raised for forage.