Sometimes the default tax rates don't apply to certain products. For example, certain types of children's clothing might be exempt from tax or have a lower tax rate. If the default rates don't apply, then you need to create an override.
Creating an override is a two-step process. You first create a collection that includes the products that have a different tax rate. Then you specify in what region the override applies and its rate.
For details on collections, refer to Collections.