Here you can specify which taxes will apply to the currently edited product. To add tax classes, click the Add tax classes button and check the boxes next to the appropriate tax classes in the displayed selection dialog. Tax classes can be removed from the product at any time using the checkboxes next to the listed items, together with the Remove selected button. You can also remove all listed items at once by clicking the icon and performing the Remove all action.
More resources concerning tax classes can be found in E-commerce Guide -> Managing your store -> Tax classes.
Further information about products can be found in E-commerce Guide -> Managing your store -> Products.