The DIM weight refers to the dimensions of the package and how they are used in the calculation of the weight of a package based on size rather than the measured "gross weight". To price international shipping, carriers charge the greater of either the actual weight or DIM weight based on the size of the box.
This means, as an example, that a 2lb package that has large dimensions like 18x30x4, like a pillow, for instance, may actually be charged at a weight of 13 lbs depending on the carrier. DIM weight calculations vary by carrier.
You can find below tools to calculate dimensional weight from DHL and FedEx;