Value added tax, VAT, is a tax on spending. Each business in a supply chain charges it on what it sells and deducts the VAT it paid on what it bought, so tax is collected a slice at a time on the value each stage adds. The European Commission describes it as collected fractionally at every stage but borne by the final consumer, not by businesses.
Rates are set country by country. In the UK the standard rate is 20%, with a reduced rate of 5% and a zero rate for things such as most food and children's clothes, according to GOV.UK. EU countries must apply a standard rate of at least 15% and may set reduced rates down to 5% for listed goods and services, under the rules summarised by the Commission. Japan's consumption tax works on the same principle.
Consumer prices in VAT countries include the tax, so the useful sum is taking it back out: divide the price by one plus the rate. A £120 price at the UK's 20% is £100 of revenue and £20 of VAT, which belongs to the tax authority, not to you. US sales tax is different in kind: it is charged once, at the final sale, and added on top of the shelf price rather than built into it.
For a registered seller VAT is a pass-through, which is why profit and margin should be worked on prices without it. It also changes marketplace fees. Amazon UK charges its referral fee on the price excluding taxes, according to its seller fee page, so on that £120 item a 15% fee is £15, not £18.
The Amazon FBA calculator strips the store's VAT rate out of the price before the referral fee when you say you are registered. The margin calculator asks you to enter cost and price on the same basis, both with VAT or both without.