Double taxation is a process in which two countries or a government entity, tax the same product, or income twice. For example in the case of income taxes that can be paid twice for the same source of income. How? Because the income is taxed at both the corporate and personal level.
If that wasn't absurd enough, then there is the concept of even more taxes that exceedes just the simple two taxes. There is triple taxation, quadruple taxation, quintuple and more.
Multiple taxation of the same product
Let's look at a bar of Snickers as an example. As it leaves its factory the first time it will get taxed is when it enters its country of destination. The second time the same Snickers bar get's taxed again is, when the wholesaler sells it to the retailer. And finally, the third time the same Snickers bar get's taxed is, when a customer buys it in a store.
This is just one example but it happens everywhere in business where selling products is involved.