Marginal tax is the tax you pay on your most recently earned SEK 1,000. As long as your overall income remains below the limit for national income tax, you pay approximately SEK 300 in tax on every SEK 1,000 earned.
On the other hand, when your overall income exceeds the limit for national income tax, you pay approximately SEK 500 in tax for every additional thousand SEK 1,000 earned . This is usually referred to as a raise in marginal tax.
The simplest way of finding out your own marginal tax is to check the tax schedule for how much tax is withdrawn from your current salary and compare it with how much tax would be levied if your salary were to be raised by, for example, SEK 1,000.