What do you mean by tax base?

The thing or amount to which a tax rate applies. To collect INCOME TAX, for example, you need a meaningful definition of INCOME.
Income that is liable to be taxed by the government. A tax base is used to determine your company’s tax liabilities. Your tax liabilities determine how much tax you owe to the relevant tax authority, in most cases, the government
Definitions of the tax base can vary enormously, over time and among countries, especially when tax breaks are taken into account. As a result, a country with a comparatively high tax rate may not have a high TAX BURDEN if it has a more narrowly defined tax base than other countries.