Any tax rates that you are currently serving. A tax rate must be defined here before it can be used on any invoice. Once a tax rate is defined here you can set it as the tax rate for clients (Client Profile -> Additional tab).
You can add a Tax Rate by going to the Settings menu and the Tax Rate sub menu.
Click on the red + icon on top right of the table to add a new tax rate. You can define the tax percentage rate and the name of the tax.