Tare is what the container or vehicle weighs with nothing inside. On a sea container it is on the door plate, together with the maximum gross mass. On a truck, it is the kerb weight shown on the registration document.
Tare matters in two calculations: the VGM by Method 2 (cargo plus the container tare) and a truck’s total mass (tractor and trailer tare plus cargo) so as not to exceed the maximum authorised mass. Mixing up gross weight and tare is a classic mistake.