Double taxation occurs when the same income is taxed twice. There are two main types. Juridical double taxation happens when two countries both tax the same person on the same income, for example when a freelancer resident in Italy earns fees from a German client and both jurisdictions claim taxing rights. Economic double taxation happens when the same income is taxed in two hands, most classically company profits taxed at the corporate level and again as dividends in the shareholder's return.
Countries relieve cross-border double taxation through a network of bilateral tax treaties based largely on the OECD Model Convention. Relief usually comes in one of two forms: the exemption method, where one country simply does not tax the foreign income, or the credit method, where your home country taxes the income but grants a credit for tax already paid abroad.
Example: you owe 3,000 of tax at home on foreign income and already paid 1,800 abroad. Under the credit method you pay only the 1,200 difference at home, so total tax is 3,000 rather than 4,800.
Your exposure depends heavily on where you are resident. See tax residency, and compare headline rates with our compare taxes by country tool. Related: capital gains tax.